Method for reading data from nonvolatile storage element, and nonvolatile storage device

ABSTRACT

Provided is a method for reading data from a variable resistance nonvolatile storage element, where the operation for reading data is less susceptible to a fluctuation phenomenon of resistance values in reading the data. The method includes: detecting a current value I read  that flows through the nonvolatile storage element that can be in a low resistance state RL and a high resistance state RH, with application of a fixed voltage; and determining that (i) the nonvolatile storage element is in a high resistance state when the current value I read  detected in the detecting is smaller than a current reference level Iref, and (ii) the nonvolatile storage element is in a low resistance state when the current value I read  detected in the detecting is larger than the reference level Iref, the current reference level Iref being defined by (IRL+IRH)/2&lt;Iref&lt;IRL.

TECHNICAL FIELD

The present invention relates to a method for reading data from avariable resistance nonvolatile storage element in which a resistancevalue changes according to an electrical signal to be applied, and to anonvolatile storage device.

BACKGROUND ART

With the development in digital technologies in recent years, electronicdevices such as mobile information equipment and information homeappliances have higher functionality. Thus, demands for increasing thecapacity of nonvolatile storage elements included in these devices,reducing the write power, accelerating writing and reading operations,and increasing the life span of these devices have been increasing.

To meet such demands, miniaturization of flash memories includingexisting floating gates is said to have limitations. Thus, attention isrecently focused on a new variable resistance nonvolatile storageelement including a variable resistance layer as a material of a storageunit.

The variable resistance nonvolatile storage element has a very simplestructure including a variable resistance layer that is disposed betweena lower electrode and an upper electrode. A resistance state of thenonvolatile storage element changes between a low resistance state and ahigh resistance state only with application, between the lower electrodeand the upper electrode, of a predetermined electric pulse having avoltage higher than or equal to a threshold. Then, information isrecorded in association with these different resistance states and itsvalues. Since the variable resistance nonvolatile storage element (alsoreferred to as “variable resistance element”) has such a simplestructure and simply performs operations, it is expected that thenonvolatile storage element can further be miniaturized and the cost canbe reduced. Since the resistance state of the variable resistancenonvolatile storage element sometimes changes between the low resistancestate and the high resistance state by orders of magnitude not longerthan 100 nanoseconds (ns), the attention is further focused on thevariable resistance nonvolatile storage elements in view of its higheroperating speed, and various proposals of these have been made.

In particular in recent years, there are many proposals for variableresistance nonvolatile storage elements comprising metal oxides in thevariable resistance layers. Such variable resistance nonvolatile storageelements comprising metal oxides can be largely divided into two types,depending on a material to be used in each of the variable resistancelayers.

One type is the variable resistance nonvolatile storage elementscomprising perovskite materials (Pr_((1-x))Ca_(x)MnO₃ [PCMO], LaSrMnO₃[LSMO], and GdBaCo_(x)O_(y) [GBCO], for example) in the variableresistance layers, as disclosed in PTL 1 and others.

The other is the variable resistance nonvolatile storage elements thatare compounds comprising only transition metals and oxygen, using binarytransition metal oxides. Compared to the perovskite materials, thebinary transition metal oxides have very simple composition structures.Thus, controlling the compositions when manufactured and in forming thefilms are relatively easy. In addition, with the advantage of relativelyfavorable compatibility with semiconductor manufacturing processes, thevariable resistance nonvolatile storage elements have intensely beenstudied in recent years. For example, PTL 2 discloses variableresistance elements comprising, as variable resistance materials, (i)transition metal oxides of stoichiometric composition, such as nickel(Ni), niobium (Nb), titanium (Ti), zirconium (Zr), hafnium (Hf), cobalt(Co), iron (Fe), copper (Cu), and chrome (Cr), and (ii) oxides whosecomposition is deficient in oxygen compared to its stoichiometriccomposition (hereinafter referred to as oxygen-deficient oxides).Furthermore, PTL 3 discloses a nonvolatile storage element comprising anoxygen-deficient tantalum (Ta) oxide as a variable resistance material.When a Ta oxide layer is denoted as TaO_(x), PTL 3 reports a resistancechange phenomenon in a range satisfying 0.8=x≦1.9 (from 44.4 to 65.5% interms of oxygen concentration).

Furthermore, PTL 3 also reports that a variable resistance nonvolatilestorage element has two different operation modes, namely, unipolar(monopolar) switching and bipolar switching.

The unipolar switching is an operation mode in which a resistance valuechanges with application of electric pulses having the same polarity anddifferent amplitudes between a lower electrode and an upper electrode ofa variable resistance nonvolatile storage element, which is disclosed byPTL 2 and others. Furthermore, the unipolar switching requires changingnot only the magnitude of the voltage but also the length (pulse width)of the electric pulse simultaneously, as disclosed by PTL 4 in detail.For example, the unipolar switching requires using two types of electricpulses, that is, a nanosecond pulse and a microsecond pulse.

In contrast, the bipolar switching is an operation mode in which aresistance value changes with application of electric pulses of positiveand negative polarities between a lower electrode and an upper electrodeof a variable resistance nonvolatile storage element, which is disclosedby PTLs 1 and 2. As disclosed by PTL 4, electric pulses of a nonvolatilestorage element that performs the bipolar switching are generally set tothe same length of order of nanoseconds.

CITATION LIST Patent Literature

[PTL 1] Japanese Unexamined Patent Application Publication No.2005-340806

[PTL 2] Japanese Unexamined Patent Application Publication No.2006-140464

[PTL 3] International Publication WO2008/059701

[PTL 4] Japanese Patent No. 4203532

SUMMARY OF INVENTION Technical Problem

As the name suggests, the nonvolatile storage element is an elementhaving characteristics of holding information without volatilizing(losing, deteriorating, and changing) the information, after theinformation is electrically stored thereon and even when the power isturned off. Thus, one of the important characteristics necessary for thenonvolatile storage element is higher holding capability of information.In other words, the nonvolatile storage element needs to have capabilityof storing information without deteriorating the information oncewritten therein. Thus, the higher information holding capability isdesired. Generally, any nonvolatile storage elements cannot avoid asituation where storage information changes within a finite length oftime.

The phenomena in which information once stored gradually changes withthe passage of time include a phenomenon in which the resistance valueof a nonvolatile storage element changes in a long period of time (forexample, over 100 hours), and a phenomenon in which the resistance valueof a nonvolatile storage element changes in a short period of time (forexample, within several minutes) (this phenomenon is referred to as“fluctuation”). In particular, any methods for effectively suppressingthe “fluctuation” phenomenon in which the resistance value of anonvolatile storage element changes in a short period of time are notsuggested yet.

The present invention has been conceived in view of the abovecircumstances, and has an object of suggesting a method for reading datathat is less susceptible to a fluctuation phenomenon of resistancevalues in reading the data, and improving the holding capability ofinformation of the nonvolatile storage element.

Solution To Problem

An aspect of the present invention to achieve the object is a method forreading data from a variable resistance nonvolatile storage element (i)including a first electrode, a second electrode, and a variableresistance layer disposed between and in contact with the firstelectrode and the second electrode and (ii) having characteristics inwhich a resistance state between the first electrode and the secondelectrode with application of a voltage having a first polarity betweenthe first electrode and the second electrode becomes a first resistancestate RL, and in which the resistance state between the first electrodeand the second electrode with application of a voltage having a secondpolarity different from the first polarity between the first electrodeand the second electrode becomes a second resistance state RH, thesecond resistance state RH>the first resistance state RL, the methodincludes: detecting a current that flows through the nonvolatile storageelement with application of a fixed voltage; and determining that (i)the nonvolatile storage element is in a high resistance state when thecurrent detected in the detecting is smaller than a current referencelevel Iref, and (ii) the nonvolatile storage element is in a lowresistance state when the current detected in the detecting is largerthan the reference level Iref, the current reference level Iref beingdefined by (IRL+IRH)/2<Iref<IRL, where IRL denotes a current that flowsthrough the nonvolatile storage element in the first resistance state RLwith application of the fixed voltage, IRH denotes a current that flowsthrough the nonvolatile storage element in the second resistance stateRH, and IRH<IRL.

Furthermore, another aspect of the present invention is a method forreading data from a variable resistance nonvolatile storage element (i)including a first electrode, a second electrode, and a variableresistance layer disposed between and in contact with the firstelectrode and the second electrode and (ii) having characteristics inwhich a resistance state between the first electrode and the secondelectrode with application of a voltage having a first polarity betweenthe first electrode and the second electrode becomes a first resistancestate RL, and in which the resistance state between the first electrodeand the second electrode with application of a voltage having a secondpolarity different from the first polarity between the first electrodeand the second electrode becomes a second resistance state RH, thesecond resistance state RH>the first resistance state RL, the methodincludes: detecting a resistance value of the nonvolatile storageelement; and determining that (i) the nonvolatile storage element is ina low resistance state when the resistance value detected in thedetecting is smaller than a resistance reference level Rref, and (ii)the nonvolatile storage element is in a high resistance state when theresistance value detected in the detecting is larger than the resistancereference level Rref, the resistance reference level Rref being definedby RL<Rref<(RL+RH)/2.

Furthermore, another aspect of the present invention to achieve theobject is a nonvolatile storage device including: a variable resistancenonvolatile storage element; and a control unit configured to read datafrom the nonvolatile storage element, wherein the nonvolatile storageelement (i) includes a first electrode, a second electrode, and avariable resistance layer disposed between and in contact with the firstelectrode and the second electrode and (ii) has characteristics in whicha resistance state between the first electrode and the second electrodewith application of a voltage having a first polarity between the firstelectrode and the second electrode becomes a first resistance state RL,and in which the resistance state between the first electrode and thesecond electrode with application of a voltage having a second polaritydifferent from the first polarity between the first electrode and thesecond electrode becomes a second resistance state RH, the secondresistance state RH>the first resistance state RL, and the control unitis configured to: detect a current that flows through the nonvolatilestorage element with application of a fixed voltage; and determine that(i) the nonvolatile storage element is in a high resistance state whenthe detected current is smaller than a current reference level Iref, and(ii) the nonvolatile storage element is in a low resistance state whenthe detected current is larger than the reference level Iref, thecurrent reference level Iref being defined by (IRL+IRH)/2<Iref<IRL,where IRL denotes a current that flows through the nonvolatile storageelement in the first resistance state RL with application of the fixedvoltage, IRH denotes a current that flows through the nonvolatilestorage element in the second resistance state RH, and IRH<IRL.

Furthermore, another aspect of the present invention is a nonvolatilestorage device including: a variable resistance nonvolatile storageelement; and a control unit configured to read data from the nonvolatilestorage element, wherein the nonvolatile storage element (i) includes afirst electrode, a second electrode, and a variable resistance layerdisposed between and in contact with the first electrode and the secondelectrode and (ii) has characteristics in which a resistance statebetween the first electrode and the second electrode with application ofa voltage having a first polarity between the first electrode and thesecond electrode becomes a first resistance state RL, and in which theresistance state between the first electrode and the second electrodewith application of a voltage having a second polarity different fromthe first polarity between the first electrode and the second electrodebecomes a second resistance state RH, the second resistance state RH>thefirst resistance state RL, the control unit is configured to: detect aresistance value of the nonvolatile storage element; and determine that(i) the nonvolatile storage element is in a low resistance state whenthe detected resistance value is smaller than a resistance referencelevel Rref, and (ii) the nonvolatile storage element is in a highresistance state when the detected resistance value is larger than theresistance reference level Rref, the resistance reference level Rrefbeing defined by RL<Rref<(RL+RH)/2.

Advantageous Effects of Invention

The method for reading data from a nonvolatile storage element and anonvolatile storage device according to the present invention can reducethe influence of the fluctuations on a set resistance value (variationin resistance value for a short period of time), and consequently,improve the information holding capability of the variable resistancenonvolatile storage element.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a cross-sectional view of a structure of anonvolatile storage element according to Embodiment 1 in the presentinvention.

FIG. 2 illustrates a circuit diagram when a voltage pulse is applied toa nonvolatile storage element according to Embodiment 1.

FIG. 3 illustrates the variation in resistance value of a nonvolatilestorage element according to Embodiment 1.

FIG. 4 shows the plotted maximum values and minimum values of variationin resistance value of a nonvolatile storage element in the highresistance state according to Embodiment 1.

FIG. 5 shows the plotted maximum values and minimum values of variationin resistance value of a nonvolatile storage element in the lowresistance state according to Embodiment 1.

FIG. 6 shows the plotted maximum values of variation in resistance valueof a nonvolatile storage element in the high resistance state accordingto Embodiment 1.

FIG. 7 shows the plotted maximum values of variation in resistance valueof a nonvolatile storage element in the high resistance state accordingto Embodiment 1.

FIG. 8 is a diagram for describing the influence of a resistancevariation phenomenon of a nonvolatile storage element according toEmbodiment 1.

FIG. 9 is a diagram for describing the influence of a resistancevariation phenomenon of a nonvolatile storage element according toEmbodiment 1.

FIG. 10 is a diagram for describing the influence of a resistancevariation phenomenon of a nonvolatile storage device according toEmbodiment 1.

FIG. 11 shows the plotted minimum values of variation in current valueof a nonvolatile storage element in the high resistance state accordingto Embodiment 2 in the present invention.

FIG. 12 is a diagram for describing the influence of a resistancevariation phenomenon of a nonvolatile storage element according toEmbodiment 2.

FIG. 13 is a diagram for describing the influence of a resistancevariation phenomenon of a nonvolatile storage element according toEmbodiment 2.

FIG. 14 is a diagram for describing the influence of a resistancevariation phenomenon of a nonvolatile storage element according toEmbodiment 2.

FIG. 15 is a block diagram illustrating an example configuration of anonvolatile storage device according to Embodiment 3 in the presentinvention.

(a) of FIG. 16 is a flow chart showing the main procedure for readingdata based on a current value of a nonvolatile storage element, and (b)of FIG, 16 is a flow chart showing the main procedure for reading databased on a resistance value of a nonvolatile storage element.

FIG. 17 is a block diagram illustrating an example configuration of anonvolatile storage device according to Embodiment 4 in the presentinvention.

FIG. 18 shows the variation (fluctuations) in resistance value of avariable resistance element.

DESCRIPTION OF EMBODIMENTS

Before describing Embodiments of the present invention, the knowledgefound by the Inventors from the experiments will be described in detail.The knowledge will be hereinafter described with reference to FIG. 18,which will be of some help to understand Embodiments of the presentinvention to be described later. Thus, the present invention is notlimited by the drawing and the description.

Generally, any nonvolatile storage elements cannot avoid a situationwhere storage information changes within a finite length of time. Thevariable resistance nonvolatile storage element according to the presentinvention is no exception where it has characteristics that informationonce stored gradually changes with the passage of time. Here, the changein information is observed as change of a set resistance value with thepassage of time. Generally, a phenomenon in which storage information isdeteriorated according to gradual change from a high resistance state toa low resistance state or from a low resistance state to a highresistance state during a certain period of time is known.

The Inventors have found a new change phenomenon in which a resistancevalue increases or decrease for a short period of time in addition tothe deterioration (deterioration in retention capability) in informationto a certain degree according to gradual change in the resistance valuefor a long period of time (for example, over 100 hours). This phenomenonis observed when a nonvolatile storage element comprising a tantalum(Ta) oxide as a variable resistance material is set to a high resistancestate, and is a phenomenon in which a set resistance value randomlyvaries during a short period of time, that is, within several minutes.The variable resistance nonvolatile storage element using a Ni oxide andhaving the similar change phenomenon in resistance value is reported(NPL 1: Daniele Ielmini et. al, Appl. Phys. Lett. Vol. 96, 2010, page53503). Thus, the phenomenon seems a general phenomenon in the variableresistance nonvolatile storage element.

An example of the phenomenon will be hereinafter described. TheInventors have manufactured a nonvolatile storage element comprising anoxygen-deficient Ta oxide as a variable resistance material, operated itwith application of electric pulses, and studied in detail how the setresistance value changed with the passage of time. The nonvolatilestorage element is a variable resistance nonvolatile storage elementhaving bipolar switching characteristics in which the state of thenonvolatile storage element is changed to a high resistance state withapplication of a positive voltage to the upper electrode with respect tothe lower electrode whereas it is changed to a low resistance state withapplication of a negative voltage to the upper electrode with respect tothe lower electrode.

FIG. 18 shows the result of measurement. The data was obtained when anelectric pulse of +2.5 V with a pulse width of 100 ns and an electricpulse of −2.0 V with a pulse width of 100 ns were alternately applied100 times in total to the prepared nonvolatile storage element connectedin series with a load resistor with a resistance of 6.4 kΩ. The electricpulse of +2.5 V with the pulse width of 100 ns was finally applied toset the nonvolatile storage element to the high resistance state(approximately to a resistance value of 120 kΩ). The nonvolatile storageelement was retained at a room temperature, and how the set resistancevalue (resistance value of the nonvolatile storage element then) changedwith the passage of time was studied. FIG. 18 shows that although thenonvolatile storage element was retained at a room temperature and avoltage not enough to cause the resistance change is applied to thenonvolatile storage element, the resistance value of the nonvolatilestorage element repeatedly and abruptly increases or decreases. In otherwords, the resistance value abruptly decreases to approximately 50 kΩafter first 200 seconds, then starts to increase after 1000 seconds, andreaches 200 kΩ. Afterward, the nonvolatile storage element hasrelatively a stable transition. In other words, the resistance valuesupposed to set to 120 kΩ largely increases or decreases for a shortperiod of time, and the resistance value changes at each measurementtiming.

The variable resistance nonvolatile storage element basically storesinformation in association with the size of a resistance value and data.There are several methods for reading data. Examples of the methodsinclude a method for measuring a resistance value of the element itselfand a method for measuring a current that flows through the element.However, in any of the cases, when a resistance state (data) is readfrom the element that is set to the resistance state, a predeterminedthreshold (determination point of data, or reference level) is set, andthe data is judged according to whether or not a read physical amount(resistance value of the nonvolatile storage element, or current valueof a current that flows through the nonvolatile storage element) islarger or smaller than the threshold.

For example, when the physical amount to be used for reading data fromthe nonvolatile storage element is defined as a resistance value, astate in which the nonvolatile storage element has a resistance valuemore (or higher) than or equal to a predetermined threshold is definedas a high resistance state, and a state in which the nonvolatile storageelement has a resistance value less (or lower) than or equal to thepredetermined threshold is defined as a low resistance state. Forexample, information is stored by allocating one of data “1” and data“0” to each of the resistance states.

However, information may be erroneously read with variation in theresistance as shown in FIG. 18. For example, assume a case where thenonvolatile storage element whose result is shown in FIG. 18 is set tothe resistance value of 120 kΩ, and its threshold is set to the half of60 kΩ. In other words, a resistance state higher than 60 kΩ is definedas a high resistance state, and a resistance state lower than 60 kΩ isdefined as a low resistance state. When a resistance value of thenonvolatile storage element is read approximately at 1000 seconds aftersetting the resistance value, the resistance value is 50 kΩ, whichindicates that the nonvolatile storage element is determined to be inthe low resistance state. However, when the resistance value of thenonvolatile storage element read after 2000 seconds exceeds 200 kΩ, itis determined that the nonvolatile storage element is in the highresistance state. In other words, depending on the timing at which dataof the nonvolatile storage element that is not rewritten is read, thedata is identified by “1” or “0”. Such a phenomenon that characterizesthe nonvolatile storage element that stores information possibly leadsto a fatal failure, and should be desirably prevented. However, there isno effective method disclosed for suppressing the phenomenon itself inwhich the resistance value varies as shown in FIG. 18.

The resistance change for a short period of time as observed in FIG. 18is distinguished from the phenomenon for a long period of time, and isexpressed as “fluctuations in resistance value” or simply“fluctuations”.

Various embodiments of reading data from a nonvolatile storage elementaccording to the present invention have been conceived based on theknowledge, and completed.

An aspect of the present invention is a method for reading data from avariable resistance nonvolatile storage element (i) including a firstelectrode, a second electrode, and a variable resistance layer disposedbetween and in contact with the first electrode and the second electrodeand (ii) having characteristics in which a resistance state between thefirst electrode and the second electrode with application of a voltagehaving a first polarity between the first electrode and the secondelectrode becomes a first resistance state RL, and in which theresistance state between the first electrode and the second electrodewith application of a voltage having a second polarity different fromthe first polarity between the first electrode and the second electrodebecomes a second resistance state RH, the second resistance state RH>thefirst resistance state RL, the method includes: detecting a current thatflows through the nonvolatile storage element with application of afixed voltage; and determining that (i) the nonvolatile storage elementis in a high resistance state when the current detected in the detectingis smaller than a current reference level Iref, and (ii) the nonvolatilestorage element is in a low resistance state when the current detectedin the detecting is larger than the reference level Iref, the currentreference level Iref being defined by (IRL+IRH)/2<Iref<IRL, where IRLdenotes a current that flows through the nonvolatile storage element inthe first resistance state RL with application of the fixed voltage, IRHdenotes a current that flows through the nonvolatile storage element inthe second resistance state RH, and IRH<IRL.

More specifically, the nonvolatile storage element is an element havingfluctuations that are characteristics in which a resistance value of thenonvolatile storage element in the second resistance state RH randomlychanges with passage of time.

Thus, the current reference level to be referenced for determining oneof the low resistance state and the high resistance state of thenonvolatile storage element is set to a value closer to a current valueof a current that flows in the low resistance state with application ofa fixed voltage than a median value between the current value of thecurrent that flows in the low resistance state and a current value of acurrent that flows in the high resistance state with application of afixed voltage. Thus, the occurrence frequency of an error in readingdata that is caused by the larger fluctuation in the resistance value ofthe nonvolatile storage element in the high resistance state is reducedmore than by the fluctuation in the resistance value of the nonvolatilestorage element in the low resistance state.

Here, in the determining, a current value larger than an average valueof the fluctuations by at least 4σ is preferably determined as thecurrent reference level Iref satisfying (IRL+IRH)/2<Iref<IRL, where σdenotes a standard deviation in the fluctuations of the current valueIRH of the nonvolatile storage element in the second resistance stateRH. Accordingly, it is possible to accurately determine most of thenonvolatile storage elements in the high resistance state to be in thehigh resistance state.

Furthermore, another aspect of the present invention is a method forreading data from a variable resistance nonvolatile storage element (i)including a first electrode, a second electrode, and a variableresistance layer disposed between and in contact with the firstelectrode and the second electrode and (ii) having characteristics inwhich a resistance state between the first electrode and the secondelectrode with application of a voltage having a first polarity betweenthe first electrode and the second electrode becomes a first resistancestate RL, and in which the resistance state between the first electrodeand the second electrode with application of a voltage having a secondpolarity different from the first polarity between the first electrodeand the second electrode becomes a second resistance state RH, thesecond resistance state RH>the first resistance state RL, the methodincludes: detecting a resistance value of the nonvolatile storageelement; and determining that (i) the nonvolatile storage element is ina low resistance state when the resistance value detected in thedetecting is smaller than a resistance reference level Rref, and (ii)the nonvolatile storage element is in a high resistance state when theresistance value detected in the detecting is larger than the resistancereference level Rref, the resistance reference level Rref being definedby RL<Rref<(RL+RH)/2.

More specifically, the nonvolatile storage element is an element havingfluctuations that are characteristics in which a resistance value of thenonvolatile storage element in the second resistance state RH randomlychanges with passage of time.

Thus, the resistance reference level to be referenced for determiningone of the low resistance state and the high resistance state of thenonvolatile storage element is set to a value closer to a resistancevalue of the nonvolatile storage element in the low resistance statethan a median value between the resistance value of the nonvolatilestorage element in the low resistance state and a resistance value ofthe nonvolatile storage element in the high resistance state. Thus, theoccurrence frequency of an error in reading data that is caused by thelarger fluctuation in the resistance value of the nonvolatile storageelement in the high resistance state is reduced more than by thefluctuation in the resistance value of the nonvolatile storage elementin the low resistance state.

Here, in the determining, a resistance value smaller than an averagevalue of the fluctuations by at least 4σ is preferably determined as theresistance reference level Rref satisfying RL<Rref<(RL+RH)/2, where σdenotes a standard deviation in the fluctuations of the resistance valueof the nonvolatile storage element in the second resistance state RH.Accordingly, it is possible to accurately determine most of thenonvolatile storage elements in the high resistance state to be in thehigh resistance state.

Here, as an example of a material of the nonvolatile storage element,the variable resistance layer preferably has a stacked structureincluding (i) a first transition metal oxide comprising a firsttransition metal and (ii) a second transition metal oxide comprising asecond transition metal, the first transition metal oxide being higherin oxygen deficiency than the second transition metal oxide.Furthermore, a condition that the second transition metal oxide islarger in resistance value than the first transition metal oxide may besatisfied.

Here, the first transition metal oxide may be identical to the secondtransition metal oxide. For example, the first transition metal oxideand the second transition metal oxide may comprise tantalum.

Furthermore, the first transition metal oxide may be different from thesecond transition metal oxide.

The second transition metal oxide is preferably lower in standardelectrode potential than the first transition metal oxide.

Furthermore, another aspect of the present invention to achieve theobject is a nonvolatile storage device including: a variable resistancenonvolatile storage element; and a control unit configured to read datafrom the nonvolatile storage element, wherein the nonvolatile storageelement (i) includes a first electrode, a second electrode, and avariable resistance layer disposed between and in contact with the firstelectrode and the second electrode and (ii) has characteristics in whicha resistance state between the first electrode and the second electrodewith application of a voltage having a first polarity between the firstelectrode and the second electrode becomes a first resistance state RL,and in which the resistance state between the first electrode and thesecond electrode with application of a voltage having a second polaritydifferent from the first polarity between the first electrode and thesecond electrode becomes a second resistance state RH, the secondresistance state RH>the first resistance state RL, and the control unitis configured to: detect a current that flows through the nonvolatilestorage element with application of a fixed voltage; and determine that(i) the nonvolatile storage element is in a high resistance state whenthe detected current is smaller than a current reference level Iref, and(ii) the nonvolatile storage element is in a low resistance state whenthe detected current is larger than the reference level Iref, thecurrent reference level Iref being defined by (IRL+IRH)/2<Iref<IRL,where IRL denotes a current that flows through the nonvolatile storageelement in the first resistance state RL with application of the fixedvoltage, IRH denotes a current that flows through the nonvolatilestorage element in the second resistance state RH, and IRH<IRL.

More specifically, the nonvolatile storage element is an element havingfluctuations that are characteristics in which a resistance value of thenonvolatile storage element in the second resistance state RH randomlychanges with passage of time.

Thus, the current reference level to be referenced for determining oneof the low resistance state and the high resistance state of thenonvolatile storage element is set to a value closer to a current valueof a current that flows in the low resistance state with application ofa fixed voltage than a median value between the current value of thecurrent that flows in the low resistance state and a current value of acurrent that flows in the high resistance state with application of afixed voltage. Thus, the occurrence frequency of an error in readingdata that is caused by the larger fluctuation in the resistance value ofthe nonvolatile storage element in the high resistance state is reducedmore than by the fluctuation in the resistance value of the nonvolatilestorage element in the low resistance state.

Here, the control unit is preferably configured to determine a currentvalue larger than an average value of the fluctuations by at least 4σ asthe current reference level Iref satisfying (IRL+IRH)/2<Iref<IRL, whereσ denotes a standard deviation in the fluctuations of the current valueIRH of the nonvolatile storage element in the second resistance stateRH. Accordingly, it is possible to accurately determine most of thenonvolatile storage elements in the high resistance state to be in thehigh resistance state.

Furthermore, another aspect of the present invention is a nonvolatilestorage device including: a variable resistance nonvolatile storageelement; and a control unit configured to read data from the nonvolatilestorage element, wherein the nonvolatile storage element (i) includes afirst electrode, a second electrode, and a variable resistance layerdisposed between and in contact with the first electrode and the secondelectrode and (ii) has characteristics in which a resistance statebetween the first electrode and the second electrode with application ofa voltage having a first polarity between the first electrode and thesecond electrode becomes a first resistance state RL, and in which theresistance state between the first electrode and the second electrodewith application of a voltage having a second polarity different fromthe first polarity between the first electrode and the second electrodebecomes a second resistance state RH, the second resistance state RH>thefirst resistance state RL, the control unit is configured to: detect aresistance value of the nonvolatile storage element; and determine that(i) the nonvolatile storage element is in a low resistance state whenthe detected resistance value is smaller than a resistance referencelevel Rref, and (ii) the nonvolatile storage element is in a highresistance state when the detected resistance value is larger than theresistance reference level Rref, the resistance reference level Rrefbeing defined by RL<Rref<(RL+RH)/2.

More specifically, the nonvolatile storage element is an element havingfluctuations that are characteristics in which a resistance value of thenonvolatile storage element in the second resistance state RH randomlychanges with passage of time.

Thus, the resistance reference level to be referenced for determiningone of the low resistance state and the high resistance state of thenonvolatile storage element is set to a value closer to a resistancevalue of the nonvolatile storage element in the low resistance statethan a median value between the resistance value of the nonvolatilestorage element in the low resistance state and a resistance value ofthe nonvolatile storage element in the high resistance state. Thus, theoccurrence frequency of an error in reading data that is caused by thelarger fluctuation in the resistance value of the nonvolatile storageelement in the high resistance state is reduced more than by thefluctuation in the resistance value of the nonvolatile storage elementin the low resistance state.

Here, the control unit is preferably configured to determine aresistance value smaller than an average value of the fluctuations by atleast 4σ as the resistance reference level Rref satisfyingRL<Rref<(RL+RH)/2, where σ denotes a standard deviation in thefluctuations of the resistance value of the nonvolatile storage elementin the second resistance state RH. Accordingly, it is possible toaccurately determine most of the nonvolatile storage elements in thehigh resistance state to be in the high resistance state.

Here, as an example of a material of the nonvolatile storage element,the variable resistance layer preferably has a stacked structureincluding (i) a first transition metal oxide comprising a firsttransition metal and (ii) a second transition metal oxide comprising asecond transition metal, the first transition metal oxide being higherin oxygen deficiency than the second transition metal oxide.Furthermore, a condition that the second transition metal oxide islarger in resistance value than the first transition metal oxide may besatisfied.

Here, the first transition metal oxide may be identical to the secondtransition metal oxide. For example, the first transition metal oxideand the second transition metal oxide may comprise tantalum.

Furthermore, the first transition metal oxide may be different from thesecond transition metal oxide.

The second transition metal oxide is preferably lower in standardelectrode potential than the first transition metal oxide.

Embodiments according to the present invention will be described withreference to the drawings. Embodiments to be described hereinafterindicate specific and preferable examples of the present invention. Thevalues, shapes, materials, constituent elements, positions andconnections of the constituent elements, steps, and orders of the stepsindicated in Embodiments are examples, and do not limit the presentinvention. Furthermore, the constituent elements in Embodiments that arenot described in independent Claims that describe the most genericconcept of the present invention are described as arbitrary constituentelements for composing more preferable embodiments.

Embodiment 1

Embodiment 1 will describe how to reduce the influence of thefluctuation phenomenon in resistance value of a variable resistancenonvolatile storage element comprising an oxygen-deficient Ta oxide in avariable resistance layer, depending on how to set a determination pointwhen the state of the nonvolatile storage element is determined betweenthe low resistance state and the high resistance state by measuring theresistance value itself. In order to do so, the following will firstdescribe a structure of a sample used and a method for manufacturing thesample, and finally how to set the determination point.

Structure of Nonvolatile Storage Element

FIG. 1 illustrates a cross-sectional view of an example structure of anonvolatile storage element according to Embodiment 1 of the presentinvention.

As illustrated in FIG. 1, a nonvolatile storage element 100 according toEmbodiment 1 includes a substrate 101, an oxide layer 102 formed on thesubstrate 101, a lower electrode 103 formed on the oxide layer 102, avariable resistance layer 106, and an upper electrode 107. Here, thevariable resistance layer 106 is a stacked layer including: a firsttransition metal oxide layer 104 comprising an oxygen-deficienttransition metal oxide; and a second transition metal oxide layer 105comprising an oxygen-deficient transition metal oxide having an oxygendeficiency lower than that of the first transition metal oxide layer104. According to Embodiment 1, the variable resistance layer 106 is astacked layer including a first oxygen-deficient tantalum oxide layer(hereinafter referred to as “first Ta oxide layer”) 104 and a secondtantalum oxide layer (hereinafter referred to as “second Ta oxidelayer”) 105. Here, the second Ta oxide layer 105 has an oxygen contentpercentage higher than that of the first Ta oxide layer 104. In otherwords, the first Ta oxide layer 104 has an oxygen deficiency higher thanthat of the second Ta oxide layer 105. The oxygen deficiency refers to aratio of deficient oxygen in a transition metal oxide, relative to theamount of oxygen included in the oxide having its stoichiometriccomposition. For example, when the transition metal is tantalum (Ta),the stoichiometric composition of the oxide is Ta₂O₅, which can beexpressed as TaO_(2.5). Here, the oxygen deficiency of TaO₂O₅ is 0%. Forexample, the oxygen deficiency of an oxygen-deficient tantalum oxidewhose composition is expressed as TaO_(1.5) is determined by; the oxygendeficiency=(2.5−1.5)/2.5=40%. The oxygen content percentage of Ta2O5 isa ratio of oxygen to the total number of atoms (O/(Ta+O)) and is thus71.4 atm %. This means that an oxygen-deficient tantalum oxide has anoxygen content percentage higher than 0 and lower than 71.4 atm %. Thesecond Ta oxide layer 105 has a resistance value (strictly speaking,specific resistance) higher than that of the first Ta oxide layer 104.

A metal included in the variable resistance layer 106 may be atransition metal other than tantalum. Usable transition metals includetantalum (Ta), titanium (Ti), hafnium (Hf), zirconium (Zr), niobium(Nb), and tungsten (W). Since the transition metal capable of taking aplurality of oxidation states may have different resistance states by anoxidation-reduction reaction. For example, it has been able to verifythat the resistance value of the variable resistance layer 106 can bestably changed at high speed in the case where a hafnium oxide is usedso that the first hafnium oxide layer 104 has a composition of HfO_(x)and the second hafnium oxide layer 105 has a composition of HfO_(y),where x is between 0.9 and 1.6 inclusive and y is larger than x invalue. In this case, the thickness of the second hafnium oxide layer 105is preferably 3 to 4 nm. Furthermore, it has been able to verify thatthe resistance value of the variable resistance layer 106 can be stablychanged at high speed in the case where a zirconium oxide is used sothat the first zirconium oxide layer 104 has a composition of ZrO_(x)and the second zirconium oxide layer 105 has a composition of ZrO_(y),where x is between 0.9 and 1.4 inclusive and y is larger than x invalue. In this case, the thickness of the second zirconium oxide layer105 is preferably 1 to 5 nm.

The first transition metal included in the first transition metal oxidelayer 104 and the second transition metal included in the secondtransition metal oxide layer 105 may be different from each other. Inthis case, it is preferable that the second transition metal oxide layer105 has a lower oxygen deficiency, that is, higher resistance, than thatof the first transition metal oxide layer 104. With such a structure, avoltage applied between the lower electrode 103 and the upper electrode107 in changing the resistance state is distributed more to the secondtransition metal oxide layer 105, which can ease the occurrence of anoxidation-reduction reaction in the second transition metal oxide layer105. Furthermore, in the case where the first transition metal and thesecond transition metal use mutually different materials, it ispreferable that the second transition metal is lower in standardelectrode potential than the first transition metal. This is because anoxidation-reduction reaction in a tiny filament (i.e., a conductivepath) formed in the second transition metal oxide layer 105 having highresistance changes the resistance value, which presumably results in theresistance change phenomenon. For example, a stable resistance changeoperation is achieved when the first transition metal oxide layer 104comprises an oxygen-deficient tantalum oxide while the second transitionmetal oxide layer 105 comprises a titanium oxide (TiO₂). Titanium (withthe standard electrode potential=−1.63 eV) is a material having a lowerstandard electrode potential than that of tantalum (with the standardelectrode potential=−0.6 eV). The standard electrode potential having alarger value represents a property of being more difficult to oxidize.Placing, in the second transition metal oxide layer 105, a metal oxidehaving a lower standard electrode potential than that of the firsttransition metal oxide layer 104 makes an oxidation-reduction reactionmore likely to occur in the second transition metal oxide layer 105.

Each resistance change phenomenon in the variable resistance layerhaving a stacked structure of the above materials presumably occurs byan oxidation-reduction reaction in a tiny filament formed in the secondtransition metal oxide layer 105 having high resistance, which changesthe resistance value. Specifically, with application of a positivevoltage to the upper electrode 107 closer to the second transition metaloxide layer 105 with respect to the lower electrode 103, oxygen ions inthe variable resistance layer 106 are attracted toward the secondtransition metal oxide layer 105. This causes an oxidation reaction inthe tiny filament formed in the second transition metal oxide layer 105,which presumably increases the resistance of the tiny filament.Conversely, with application of a negative voltage to the upperelectrode 107 closer to the second transition metal oxide layer 105 withrespect to the lower electrode 103, oxygen ions in the second transitionmetal oxide layer 105 are forced toward the first transition metal oxidelayer 104. This causes a reduction reaction in the tiny filament formedin the second transition metal oxide layer 105, which presumablydecreases the resistance of the tiny filament.

The upper electrode 107 connected to the second transition metal oxidelayer 105 having a lower oxygen deficiency comprises, for example,platinum (Pt) and iridium (Ir) which are materials each having a higherstandard electrode potential than those of transition metals included inthe second transition metal oxide layer 105 and materials included inthe lower electrode 103. Such a structure allows an oxidation-reductionreaction to selectively occur in the second transition metal oxide layer105 near the interface between the upper electrode 107 and the secondtransition metal oxide layer 105, which provides a stable resistancechange phenomenon.

When the nonvolatile storage element 100 with the structure is driven,an external power source applies a voltage that satisfies apredetermined condition, between the lower electrode 103 and the upperelectrode 107.

Method For Manufacturing Nonvolatile Storage Element

Next, a method for manufacturing the nonvolatile storage element 100according to Embodiment 1 will be described.

First, thermal oxidation produces the oxide layer 102 having a thicknessof 200 nm on the substrate 101 which is a single-crystal silicon. Then,a tantalum oxide (TaN) layer having a thickness of 40 nm is formed asthe lower electrode 103 on the oxide layer 102 using reactive sputteringwith which a Ta target is sputtered in a mixed gas of argon (Ar) andnitrogen (N₂).

Then, the first oxygen-deficient Ta oxide layer 104 is deposited on thelower electrode 103. Here, the first oxygen-deficient Ta oxide is formedusing the reactive sputtering with which a Ta target is sputtered in Arand oxygen (O₂) gas. The specific sputtering conditions when theoxygen-deficient Ta oxide is deposited are: power of 1000 W, the Ar gasflow rate of 20 sccm, the O₂ gas flow rate of 20 sccm, and the total gaspressure of 0.05 Pa. Under these conditions, the first oxygen-deficientTa oxide layer 104 having an oxygen content percentage of approximately56 atm % and the resistivity of 2 mΩcm is deposited. Furthermore, thefirst oxygen-deficient Ta oxide layer 104 has a thickness of 45 nm.

Next, the second oxygen-deficient Ta oxide layer 105 is deposited on thesurface of the first oxygen-deficient Ta oxide layer 104 by sputteringthe TaO₂ target. The sputtering conditions are power of 200 W, the Argas flow rate of 300 sccm, and the total gas pressure of 0.3 Pa.Accordingly, the second oxygen-deficient Ta oxide layer 105 having anoxygen content percentage closer to 72 atm % is deposited with athickness of 5.5 nm (the layer is provided to stabilize the initialoperation of the element, and thus, it is not always necessary toprovide the layer when the element is formed).

Then, the upper electrode 107 is formed on the second oxygen-deficientTa oxide layer 105. Here, the upper electrode 107 comprises, forexample, iridium (Ir). Specifically, the upper electrode 107 is formedwith a thickness of 50 nm by sputtering the Ir target in the Ar gas.

With the processes, the nonvolatile storage element 100 can bemanufactured in which the variable resistance layer 106 comprising anoxygen-deficient Ta oxide is disposed between the lower electrode 103and the upper electrode 107.

Setting Resistance Values

The electric pulse signal was applied between the lower electrode 103and the upper electrode 107 of the nonvolatile storage element 100manufactured as described above to cause a resistance change. The casewhere a voltage pulse is applied as an electric pulse signal will bedescribed hereinafter. As long as a current pulse other than the voltagepulse generates a voltage to be described below, any current pulse willdo.

The positive and negative polarities of a voltage are represented withrespect to the lower electrode 103. In other words, a higher voltageapplied to the upper electrode 107 with respect to the lower electrode103 is represented by a positive polarity, and conversely, a lowervoltage applied to the upper electrode 107 with respect to the lowerelectrode 103 is represented by a negative polarity. When a positivevoltage is applied to the nonvolatile storage element 100 manufacturedas described above, the state of the nonvolatile storage element 100 ischanged to the high resistance state. Conversely, when a negativevoltage is applied to the nonvolatile storage element 100, the state ofthe nonvolatile storage element 100 is changed to the low resistancestate.

According to Embodiment 1, a voltage was applied to a variableresistance nonvolatile storage element 201 (corresponding to thenonvolatile storage element 100) that is connected in series with a loadresistor 202 with various resistances of 0 to 6.4 kΩ. Specifically, theelectrical pulses of voltages of +2.5 V and −2.0 V with a length (pulsewidth) of 100 ns were alternately applied 100 times between terminals203 and 204 in FIG. 2.

There are the two reasons why the variable resistance nonvolatilestorage element 201 is connected to the load resistor 202. One is thatthe set resistance value of the nonvolatile storage element 201 changesby being connected to the load resistor 202, and information on variousresistance ranges can be obtained. The sample used in Embodiment 1 hascharacteristics in which the low resistance value of the nonvolatilestorage element 201 is equivalent to that of the load resistor 202.Furthermore, the high resistance value thereof frequently ranges between10 to 100 times of the low resistance values. Thus, when the loadresistor 202 has a low resistance value, the resistance value of thenonvolatile storage element 201 can be set lower. Conversely, when theload resistor 202 has a high resistance value, the resistance value ofthe nonvolatile storage element 201 can be set higher.

The second reason is that it is expected that the fluctuation phenomenonof resistance values may occur when the nonvolatile storage element 201is actually used. The variable resistance nonvolatile storage element isnot used alone when it is actually used, but is used in a state of beingconnected to a transistor or a diode having a certain resistance value.Otherwise, the variable resistance nonvolatile storage element has alittle more resistance due to the wiring. Thus, assuming the externalload resistance occurring in the actual use, the nonvolatile storageelement 201 was connected to the load resistor 202.

As described above, the nonvolatile storage element 201 was set to ahigh resistance state (represented by a resistance value RH) and a lowresistance state (represented by a resistance value RL). When thenonvolatile storage element 201 was set to the high resistance state,the electric pulses of +2.5 V and −2.0 V were alternatively applied 100times, and finally, the electric pulse of +2.5 V was applied once.Conversely, when the nonvolatile storage element 201 was set to the lowresistance state, filially, the electric pulse of −2.0 V was appliedonce. The pulse width was 100 ns in both cases.

Measuring Variations In Resistance Value For A Short Period of Time

With the procedure, the sample of the nonvolatile storage element 201 towhich the resistance value was set was retained at a room temperature,and the resistance value of the nonvolatile storage element 201 wasmeasured with application of a voltage of 50 mV every 20 seconds. Thenonvolatile storage element 201 had no change in the resistance valuethis time with application of the voltage of approximately 50 mV used inthe measurement. FIG. 3 shows an example of the result. Specifically,FIG. 3 shows the result of measurement of resistance values of thenonvolatile storage element up to 50,000 seconds by setting, to the highresistance state, the nonvolatile storage element connected to a loadresistor of 6.4 kΩ. The vertical axis of FIG. 3 shows the resistancevalues of a single nonvolatile storage element that are calculated bysubtracting the load resistance of 6.4 kΩ. Here, the resistance valueimmediately after the setting is approximately 170 kΩ. However, thegraph shows that the resistance values increase or decrease with thepassage of time, and the fluctuation phenomenon occurs. In other words,in the fluctuation phenomenon, the resistance value has decreased to theminimum value of 150 kΩ at approximately 2,000 seconds, and theresistance value has increased to the maximum value of 250 kΩ atapproximately 20,000 seconds after starting the measurement.

The similar measurement was conducted on nonvolatile storage elements bybeing connected to each of load resistors of 0Ω (no load), 1700Ω, 2150Ω,3850Ω, 4250Ω, and 6400Ω. FIG. 4 is a summary of the results. FIG. 4shows the plotted results of (a) the set resistance values of thenonvolatile storage elements in the horizontal axis, and (b) the maximumvalues and the minimum values of the resistance values of thenonvolatile storage elements from start of the measurement to 50,000seconds in the vertical axis. The data items represented by blackcircles indicate the maximum values, and the data items represented bywhite circles indicate the minimum values. Furthermore, FIG. 4 alsoshows the results of fitting the data items. The solid line indicatesthe result of fitting the maximum values, and the broken line indicatesthe result of fitting the minimum values.

FIG. 4 shows, for example, when the set resistance value is 100 kΩ, theresistance values range approximately between 80 kΩ and 200 kΩ onaverage with fluctuation. FIG. 4 also shows relational expressions(approximate expressions) obtained from the fittings. In the relationalexpressions, x is a set resistance value, and y is the maximum value orthe minimum value.

The similar measurement was also conducted in the low resistance state.(a) of FIG. 5 shows the result. Here, (b) of FIG. 5 shows an enlargedresult of (a) of FIG. 5 in order to easily understand a part of (a). InFIG. 5, the data items represented by black rhombic marks indicate themaximum values, and the data items represented by white rhombic marksindicate the minimum values. Furthermore, the solid line and the brokenline indicate the result of fitting the maximum values and the result offitting the minimum values, respectively. FIG. 5 shows that theresistance variations (fluctuations) in the low resistance state aresmaller than those in the high resistance state.

Influence of Variations In Resistance Value On Setting Values For AShort Period of Time

Next, using the results of FIGS. 4 and 5, how much degree and at whichprobability the resistance values fluctuate according to the variations(fluctuations) for a short period of time was estimated. The methodswill be described hereinafter.

In order to estimate the influence of the resistance variations, it isnecessary to separately estimate (i) at how much degree the resistancevalues of the nonvolatile storage elements that were set to the highresistance state vary upward (increase) and (ii) at how much degree theresistance values vary downward (decrease), or (i) at how much degreethe resistance values of the nonvolatile storage elements that were setto the low resistance state vary upward and (ii) at how much degree theresistance values vary downward.

When the data items in FIG. 4 are normally distributed as arepresentative example, the procedure for estimating at how much degreeand at which probability the resistance values of the nonvolatilestorage elements that are set to the high resistance state vary upwardor downward will be described hereinafter.

First, each of the results of measurement (value of each of the blackcircles) is divided by a corresponding result of fitting the maximumvalues of the resistance variations in the high resistance state of FIG.4 (solid line in FIG. 4). Specifically, each of the results ofmeasurement is divided by 0.6903×(set resistance value)^(1.0666) thatrepresents the result of fitting. Accordingly, the result as shown inFIG. 6 is obtained. FIG. 6 shows how many times the result ofmeasurement is larger than the result of fitting (namely, displacementfrom the result of fitting), where the horizontal axis represents theset resistance values and the vertical axis represents the results ofthe division. According to the results, the displacement from thefitting does not strongly depend on the set resistance values, and theresults of measurement are distributed with some variations. Here, thecalculated average value and standard deviation (σ) are 1.037 and 0.330,respectively.

Next, at how much degree and in which range the maximum values of theresistance values of the nonvolatile storage elements that are set tothe high resistance state are present was estimated, using the resultsof FIG. 6. More specifically, the resistance values when thedistribution is represented by (+a×σ) are calculated by:

(1.037+a×0.330)×0.6903×(set resistance value)^(1.0666)   (1).

The resistance values when the distribution is represented by −a×σ) arecalculated by:

(1.037−a×0.330)×0.6903×(set resistance value)^(1.0666)   (2).

According to a statistic theory, these values indicate that for example,in the case of a=1, 68.27% of all the data items of the resistancevalues are distributed in a range obtained by the equations (1) and (2)(that is, the external range is 31.73%), In the case of a=2, 95.45%(external range 4.55%) of all the data items are distributed. In thecase of a=3, 99.73% (external range 0.27%) of all the data items aredistributed. In the case of a=4, 99.9937% (external range 0.0063%) ofall the data items are distributed.

The calculation is intended to find out at how much degree theresistance values of the nonvolatile storage elements vary upward. Here,only the equation (1) was focused on, and how the resistance valuesrepresented by (+1×σ), (+2×σ), (+3×σ), and (+4×σ) (hereinafter denotedas (+1σ), (+2σ), (+3σ), and (+4σ, respectively) are distributed wascalculated. FIG. 7 shows the results. FIG. 7 shows that as a increases,the variation amount of the resistance values of the nonvolatile storageelements increases (however, the occurrence probability is very small).Assuming that the displacement from the fitting in FIG. 4 is normallydistributed, approximately 15.87% (=(100−68.27)/2) of all the data itemsare present above the line representing 1σ. Approximately 2.28% ispresent above the line representing 2σ, approximately 0.14% is presentabove the line representing 3σ, and approximately 0.0032% is presentabove the line representing 4σ. Although not described herein, similarcalculations are performed for the resistance values varying downward inthe high resistance state and the resistance values varying upward anddownward in the low resistance state to find out the influence ofvariations in resistance value for a short period of time.

Method For Determining One of High Resistance State And Low ResistanceState

With the procedure, it becomes possible to estimate at how much degreeand at which probability a set resistance value varies with thefluctuation phenomenon. However, since the result shown in FIG. 7 is notclear, assuming the actual operation of the variable resistancenonvolatile storage element, the specific influence of the variationphenomenon on the resistance values was estimated.

First, assume that a nonvolatile storage element has a resistance valueof 5 kΩ in the low resistance state, and a resistance value of 50 kΩ inthe high resistance state. These resistance values are typicallypossible values when the nonvolatile storage element according toEmbodiment 1 operates by being connected to a load resistor of severalkΩ. FIG. 8 is a summary of the calculated influence of the fluctuations.

In FIG. 8, when the resistance value of the nonvolatile storage elementin the high resistance state is set to 50 kΩ, the resistance variationdistributed within a range of 1σ is represented by an error bar with ablack circle. The resistance variations distributed within ranges of 2σ,3σ, and 4σ are represented by error bars with a black triangle, a blacksquare, and a black rhombus, respectively.

When the resistance value of the nonvolatile storage element in the lowresistance state is set to 5 kΩ, the distributions of resistance valuesthat vary due to the fluctuation phenomenon within the ranges of 1σ, 2σ,3σ, and 4σ are represented by error bars with a white circle, a whitetriangle, a white square, and a white rhombus, respectively.

The chart indicates that in the low resistance state, the influence ofthe fluctuations is very subtle even with consideration of thedistributions up to 4σ, and the resistance value of the nonvolatilestorage element hardly changes. On the other hand, the chart indicatesthat it is highly likely that the resistance value of the nonvolatilestorage element in the high resistance state significantly changes dueto the strong influence of the fluctuations.

Assuming that a determination point for determining one of the highresistance state and the low resistance state (determination resistancevalue or resistance reference level Rref) is set to 27.5 kΩ that is amedian value (average value) between 50 kΩ and 5 kΩ (that is,determining that the nonvolatile storage element is in the highresistance state when the determination resistance value is higher thanthe median value, and that the nonvolatile storage element is in the lowresistance state when the determination resistance value is lower thanthe median value), it is wrongly determined with a higher probabilitythat the nonvolatile storage element in the high resistance state is inthe low resistance state. As shown in FIG. 8, one of the ends of theerror bar of 2σ in the high resistance state crosses the linerepresenting the median value. This indicates that approximately 2.28%of the nonvolatile storage elements that are set to the high resistancestate (50 kΩ) are wrongly determined to be in the low resistance state.However, if the determination resistance value is set closer to the lowresistance state (5 kΩ), the probability of wrong determinationsignificantly decreases. For example, when 8 kΩ is defined as thedetermination resistance value, the determination resistance value doesnot cross the error bar representing 4σ. Thus, the probability ofwrongly determining a resistance value becomes lower than or equal to0.0032%. In other words, setting the determination resistance value to avalue closer to a value in the low resistance state makes it possible toset the probability of wrong determination in reading data toapproximately 1/1000.

The results show that the determination resistance value (resistancereference level Rref) for differentiating between the high resistancestate and the low resistance state needs to be set to a resistance valuecloser to the one in the low resistance state as much as possible than amedian value between the resistance value in the high resistance stateand the resistance value in the low resistance state, in order to reducethe probability of having an error in reading data due to thefluctuations in resistance value of the variable resistance nonvolatilestorage elements. The determination point for a resistance value isnormally set to a median value between a resistance value in the highresistance state and a resistance value in the low resistance state inmany cases to minimize the influence of variation in set current valuesthat is caused by the element variation or setting variation inresistance value. In other words, the determination point is set to themedian value based on the idea that the resistance values vary similarlyin the high resistance state and the low resistance state. However, inconsideration of the fluctuation phenomenon of the resistance values,the determination point of a resistance value (that is, resistancereference level Rref) is desirably set closer to the resistance value inthe low resistance state than the median value between the resistancevalue in the high resistance state and the resistance value in the lowresistance state.

As described above, it is clear that the following method is preferableas a method for reading data from a variable resistance nonvolatilestorage element. More specifically, the preferable method for readingdata from a variable resistance nonvolatile storage element havingcharacteristics in which a resistance state between the first electrodeand the second electrode with application of a voltage having a firstpolarity (for example, negative polarity) between the first electrodeand the second electrode becomes a first resistance state (that is,resistance value RL in a low resistance state), and in which theresistance state between the first electrode and the second electrodewith application of a voltage having a second polarity (for example,positive polarity) different from the first polarity between the firstelectrode and the second electrode becomes a second resistance state(that is, resistance value RH (>RL) in a high resistance state)includes: detecting a resistance value of the nonvolatile storageelement; and determining that (i) the nonvolatile storage element is inthe low resistance state when the resistance value detected in thedetecting is smaller than a resistance reference level Rref, and (ii)the nonvolatile storage element is in the high resistance state when theresistance value detected in the detecting is larger than the resistancereference level Rref, the resistance reference level Rref being definedas a state that is closer to a resistance value in the low resistancestate than a median value between the low resistance state and the highresistance state (for example, RL<Rref<(RL+RH)/2), Accordingly, evenwhen the nonvolatile storage element in the second resistance state hascharacteristics (fluctuations) of having random change in the resistancevalue with the passage of time, the error in reading data that is causedby the fluctuations in resistance value of the nonvolatile storageelement is avoided and consequently, the information holding capabilityof the nonvolatile storage element is improved.

Here, the specific method for determining the resistance reference levelRref may be, for example, to measure the fluctuations of the resistancevalue RH of the nonvolatile storage element in the high resistance statebeforehand and determine, as the resistance reference level Rref, areference resistance value with which the resistance value RH within therange of 4σ in the distribution of the fluctuation is determined to bein the high resistance state (that is, resistance value smaller than theaverage value of the fluctuated resistance values RH by 4σ). In otherwords, in the determining, a resistance value smaller than an averagevalue of the fluctuations by at least 4σ is preferably determined as theresistance reference level Rref satisfying RL<Rref<(RL+RH)/2, where σdenotes a standard deviation in the fluctuations of the resistance valueof the nonvolatile storage element in the second resistance state RH.

The specific method for determining RH and RL in RL<Rref<(RL+RH)/2 fordefining the range of the resistance reference level Rref may bedescribed below as an example:

Prepare a memory cell array including a plurality of nonvolatile storageelements beforehand to be described later in Embodiment 3, and calculatea fluctuation of the resistance values RH of the memory cell array inthe high resistance state (distribution of resistance values) and afluctuation of the resistance values RL of the memory cell array in thelow resistance state (distribution of resistance values). Then,determine representative values RH and RL in each of the distributions,and use these values as RH and RL for defining the range of theresistance reference level Rref.

The methods for determining the representative values include (1)determining an average value of each of the distributions as arepresentative value, (2) representing each distribution by frequencydistribution (using a horizontal axis as a resistance value and avertical axis as a frequency) and determining a resistance value thatpeaks in the frequency distribution as a representative value, and (3)representing each distribution by arranging resistance values (ascendingorder of the resistance values) and determining a resistance value to bea median in the arrangement as a representative value.

Although Embodiment 1 describes a variable resistance nonvolatilestorage element comprising a Ta oxide in the variable resistance layer,the variable resistance nonvolatile storage element is not limited tosuch. Embodiment 1 is applicable to a nonvolatile storage element havingthe variation phenomenon in resistance value for a short period of time.For example, it is possible to reduce the probability of error inreading data by determining the determination resistance value, for thenonvolatile storage element comprising an Ni oxide in the variableresistance layer as reported in NPL 1.

As described above, the resistance change phenomenon in the variableresistance layer having a stacked structure presumably occurs when theresistance value of the variable resistance layer changes by anoxidation-reduction reaction in a tiny filament formed in the secondtransition metal oxide layer 105 having high resistance. Thus, thefluctuation phenomenon found by the Inventors this time presumablyoccurs because the conduction state in the tiny filament changes due tosome influences. More specifically, it is likely that the fluctuationsoccur due to imperfectly bonding or dissociating oxygen atoms.Furthermore, it is likely that electric potentials change and theresistance state fluctuates because electrons are captured or ejected ina dangling bond in the tiny filament. Thus, whether the first transitionmetal included in the first transition metal oxide layer 104 and thesecond transition metal included in the second transition metal oxidelayer 105 are identical to or different from each other in a variableresistance element having a structure in which the resistance valueincreases or decreases according to the resistance state of the tinyfilament, it is assumed that the fluctuation phenomenon essentiallyoccurs at various degrees. Determining the determination resistancevalue (resistance reference level) for the variable resistancenonvolatile storage element having such fluctuations enables reductionin the probability of error in reading data.

Furthermore, although Embodiment 1 describes a case where the resistancevalue in the low resistance state is set to 5 kΩ and the resistancevalue in the high resistance state is set to 50 kΩ, the set resistancevalue may be others. For example, FIG. 9 shows the case where theresistance value in the low resistance state is set to 2.5 kΩ and theresistance value in the high resistance state is set to 25 kΩ, and FIG.10 shows the case where the resistance value in the low resistance stateis set to 10 kΩ and the resistance value in the high resistance state isset to 100 kΩ. In both of the cases, setting the determination pointbetween the resistance value in the low resistance state and theresistance value in the high resistance state shows that it is highlylikely that the operation for reading data is significantly susceptibleto the fluctuations. Specifically, in both of the cases, setting thedetermination point (resistance reference level Rref) to a value closerto the resistance value in the low resistance state than the medianvalue between the resistance value in the low resistance state and theresistance value in the high resistance state makes it possible toreduce the error in reading data that is caused by the influence offluctuations.

Furthermore, although the influence of the resistance variations on setresistance values was evaluated by (i) changing the values of the loadresistors connected to the nonvolatile storage elements, (ii) obtainingdata of the resistance variations in the set resistance values in a widerange for a short period of time, and (iii) evaluating the distributionof the data according to Embodiment 1, the method is not limited tosuch. For example, the method may include measuring, several times, theresistance variations in resistance value for a short period of timeunder fixed conditions of the load resistors, statistically processingthe data, and evaluating the influence of the resistance variations onthe set resistance values for a short period of time.

Embodiment 2

Embodiment 1 describes how to set a determination point of a resistancevalue to reduce the influence of the fluctuation phenomenon, when astate of a variable resistance nonvolatile storage element is determinedto be one of a low resistance state and a high resistance stateaccording to a magnitude of the resistance value. Embodiment 2 willdescribe a case where the state is determined according to a magnitudeof a current that flows through the nonvolatile storage element.

Embodiment 2 applies the nonvolatile storage element used for studyingthe fluctuation phenomenon in resistance value of the nonvolatilestorage element and the operations for studying the influence of thefluctuations on the set resistance value that are described inEmbodiment 1. In other words, the nonvolatile storage element used is anonvolatile storage element including, on a substrate 101 that is asingle-crystal silicon, an oxide layer 102 having a thickness of 200 nm,a lower electrode 103 comprising TaN and having a thickness of 40 nm, afirst oxygen-deficient Ta oxide layer 104 having a thickness of 45 nm, asecond oxygen-deficient Ta oxide layer 105 having a thickness of 5.5 nm,and an upper electrode 107 comprising Ir and having a thickness of 50nm. Various load resistors were connected to the nonvolatile storageelement, voltages of +2.5V and −2.0V were applied to the nonvolatilestorage element to cause the resistance change, and the resistance stateof the nonvolatile storage element was set to the high resistance stateand the low resistance state. Then, the variation in resistance value ofthe nonvolatile storage element for a short period of time was measuredat a room temperature, and a relationship between the variation and theset resistance value was determined as shown in FIGS. 4 and 5. Next, thedistribution of the resistance values was determined, and distributioncurves of the resistance values were determined as shown in FIG. 7.

Next, the relationship in FIG. 7 was expressed in terms of current.Here, assuming the actual use of the nonvolatile storage element, therelationship was calculated using the load resistor of 5 kΩ. Here, thereason why the load resistor of 5 kΩ was used is because of assumptionthat an ON resistance value of a transistor becomes approximately 5 kΩunder, for example, the IT1R structure in which one nonvolatile storageelement is connected to one transistor (actually, a transistor havingsuch an ON resistance value can be easily formed). Furthermore, thevoltage for reading data was 50 mV. In such a case, a value of a currentthat flows through the nonvolatile storage element can be calculated bythe following equation. In other words, when R denotes a resistancevalue of the nonvolatile storage element, the current I that flowsthrough the nonvolatile storage element can be calculated by:

I=0.050/(R+5000)(A)   (3).

FIG. 11 shows the result obtained by converting the resistance values inFIG. 7 into current values using the equation (3). The influence of thefluctuation when the minimum values of the resistance values in the highresistance state and the maximum values and the minimum values of theresistance values in the low resistance state are converted to currentvalues was also calculated using the similar procedure (not shown in thedrawings).

FIG. 12 shows an easy-to-understand summary of the influence offluctuations when the resistance values are converted to current valuesusing these results of calculation. Here, the set current value in thehigh resistance state was 0.9 μA (corresponding to a resistance value of50 kΩ), and the set current value in the low resistance state was 5 μA(corresponding to a resistance value of 5 kΩ). FIG. 12 shows that evenwhen the resistance values are converted to current values, theinfluence of the fluctuations appears more strongly in the highresistance state than the low resistance state. In other words, it ishighly likely that the current values of the nonvolatile storage elementsignificantly vary in the high resistance state due to the fluctuations.For example, when the determination point (that is, current referencelevel Iref) of a current value is set to a median value between the setcurrent value in the low resistance state and the set current value inthe high resistance state, the line indicating the median value (2.95μA) crosses the error bar indicating the distribution of 4σ in the highresistance state. Thus, the probability with which the current value inthe high resistance state exceeds the determination point (probabilityof wrongly determining the nonvolatile storage element in the highresistance state to be in the low resistance state) ranges approximatelybetween 0.0032 and 0.14%. However, setting the determination point ofthe current value to, for example, 3.6 μA enables reduction of theprobability of wrongly determining data to less than 0.0032%.Conversely, setting the determination point of the current value to 2 μAincreases the probability of wrongly determining data to more than0.14%. Thus, the determination point of the current value is desirablyset to a value closer to the current value in the low resistance stateas much as possible than the median value between the current value inthe low resistance state and the current value in the high resistancestate.

When one of the high resistance state and the low resistance state isdetermined using the current that flows through the nonvolatile storageelement, the determination point of the current value is normally set tothe median value between the current value in the low resistance stateand the current value in the high resistance state in many cases tominimize the influence of variation in set current values that is causedby the element variation or setting variation in resistance value.However, in consideration of the fluctuation phenomenon of theresistance values as described above, the determination point of acurrent value (current reference level Iref) is desirably set closer tothe current value in the low resistance state than the median valuebetween the current value in the high resistance state and the currentvalue in the low resistance state.

The fact that the probability of wrong determination using thedetermination method according to Embodiment 2 is lower than thataccording to Embodiment 1 will be described hereinafter, In other words,the fact that the method for determining whether a nonvolatile storageelement is in one of the low resistance state and the high resistancestate using a current that flows through the nonvolatile storage elementwith application of a fixed voltage (Embodiment 2) is more preferablethan that using a resistance value of the nonvolatile storage element(Embodiment 1) in view of the probability of wrong determination will bedescribed hereinafter.

When the resistance state (high resistance state or low resistancestate) of a nonvolatile storage element is determined using theresistance value (Embodiment 1), the resistance reference level Rref isdefined by an equation of (RL+RH)/2) as a median value between aresistance value in the high resistance state and a resistance value inthe low resistance state. In the example shown in FIG. 8, the error barsof 2σ, 3σ, and 4σ in the high resistance state cross the linerepresenting the median value (that is, the nonvolatile storage elementin the high resistance state may be wrongly determined to be in the lowresistance state).

When the resistance state (high resistance state or low resistancestate) of a nonvolatile storage element is determined using a currentthat flows through the nonvolatile storage element with application of afixed voltage (Embodiment 2), the current reference level Iref isdefined by an equation of (IRL+IRH)/2) as a median value between acurrent value in the high resistance state and a current value in thelow resistance state. In the example shown in FIG. 12, only the errorbar of 4σ in the high resistance state crosses the line representing themedian value (that is, the nonvolatile storage element in the highresistance state may be wrongly determined to be in the low resistancestate).

As described above, as a result of the comparison between FIGS. 8 and12, when a median value between the high resistance state and the lowresistance state is defined as a reference level, the probability ofwrong determination decreases more with the method for determining theresistance state of the nonvolatile storage element using the currentthat flows through the nonvolatile storage element than with the methodusing the resistance value.

The reason will be described in detail hereinafter.

When the resistance state of the nonvolatile storage element isdetermined using a resistance value, the median value expressed by(RL+RH)/2 serves as a measure for the resistance reference level Rref.Here, as indicated by the equation of (RL+RH)/2, the resistance value RHin the high resistance state dominantly determines a median value. Whenthe resistance state of the nonvolatile storage element is determinedusing a current value of a current that flows through the nonvolatilestorage element, the median value expressed by (IRL+IRH)/2 serves as ameasure for the current reference level Iref. Here, as indicated by theequation of (IRL+IRH)/2, the current value IRL in the low resistancestate dominantly determines a median value.

For example, consider a case where the resistance value RH in the highresistance state and the resistance value RL in the low resistance statesatisfy a relationship of RH=10×RL (that is, IRL=10×IRH). Here, theresistance value RH in the high resistance state having a largerfluctuation has a margin expressed by RH:(RL+RH)/2=10:5.5 (approximatelydouble) with respect to the median value expressed by (RL+RH)/2. On theother hand, the current value IRH in the high resistance state having alarger fluctuation has a margin expressed by IRH:(IRL+IRH)/2=1:5.5(approximately 5 to 6 times) with respect to the median value expressedby (IRL+IRH)/2. Thus, the latter has a sufficient margin for thefluctuation.

Thus, the probability of wrong determination decreases more with themethod for determining the resistance state of the nonvolatile storageelement using the current that flows through the nonvolatile storageelement (that is, defining the current reference level Iref using themedian value (IRL+IRH)/2 as a measure) than with the method using theresistance value (that is, defining the resistance reference level Rrefusing the median value (RL+RH)/2 as a measure).

As described above, it is clear that the following method is preferableas a method for reading data from a variable resistance nonvolatilestorage element. More specifically, the preferable method for readingdata from a variable resistance nonvolatile storage element havingcharacteristics in which a resistance state between the first electrodeand the second electrode with application of a voltage having a firstpolarity (for example, negative polarity) between the first electrodeand the second electrode becomes a first resistance state (that is,resistance value RL in a low resistance state), and in which theresistance state between the first electrode and the second electrodewith application of a voltage having a second polarity (for example,positive polarity) different from the first polarity between the firstelectrode and the second electrode becomes a second resistance state(that is, resistance value RH (>RL) in a high resistance state)includes: detecting a current that flows through the nonvolatile storageelement with application of a fixed voltage; and determining that (i)the nonvolatile storage element is in the high resistance state when thecurrent detected in the detecting is smaller than a current referencelevel Iref, and (ii) the nonvolatile storage element is in the lowresistance state when the current detected in the detecting is largerthan the reference level Iref, the current reference level Iref beingdefined by (IRL+IRH)/2<Iref<IRL, where IRL denotes a current that flowsthrough the nonvolatile storage element in the first resistance statewith application of the fixed voltage, IRH denotes a current that flowsthrough the nonvolatile storage element in the second resistance state,and IRH<IRL. Accordingly, even when the nonvolatile storage element inthe second resistance state has characteristics (fluctuations) of havingrandom change in the resistance value with the passage of time, theerror in reading data that is caused by the fluctuations in resistancevalue of the nonvolatile storage element is avoided and consequently,the information holding capability of the nonvolatile storage element isimproved.

Here, the specific method for determining the current reference levelIref is, for example, to measure the fluctuations of the current valueIRH of the nonvolatile storage element in the high resistance statebeforehand and determine, as the current reference level Iref, areference current value with which the current value IRH within therange of 4σ in the distribution of the fluctuations determines that thenonvolatile storage element is in the high resistance state (that is,current value larger than the average value of the fluctuated currentvalues IRH by 4σ). In other words, in the determining, a current valuelarger than an average value of the fluctuations by at least 4σ ispreferably determined as the current reference level Iref satisfying(IRL+IRH)/2<Iref<IRL, where σ denotes a standard deviation in thefluctuations of the current value IRH of the nonvolatile storage elementin the second resistance state RH.

The specific method for determining IRH and IRL in (IRL+IRH)/2<Iref<IRLfor defining the range of the current reference level Iref may bedescribed below as an example:

Prepare a memory cell array including a plurality of nonvolatile storageelements beforehand to be described later in Embodiment 3, and calculatea fluctuation of the current value IRH of the memory cell array in thehigh resistance state (distribution of current values) and a fluctuationof the current value IRL of the memory cell array in the low resistancestate (distribution of current values). Then, determine representativevalues IRH and IRL in each of the distributions, and use these values asIRH and IRL for defining the range of the current reference level Iref.

The methods for determining the representative values include (1)determining an average value in each distribution as a representativevalue, (2) representing each distribution by frequency distribution(using a horizontal axis as a resistance value and a vertical axis as afrequency) and determining a current value that peaks in the frequencydistribution as a representative value, and (3) representing eachdistribution by arranging current values (ascending order of the currentvalues) and determining a current value to be a median in thearrangement as a representative value.

The above case assumes a state in which a load resistor of 5 kΩ isconnected to the nonvolatile storage element and the read voltage (fixedapplication voltage for detecting a current value) is 50 mV. However,these values are set as examples, and values other than these valuessufficiently produce the advantage of reducing the occurrence of anerror in reading data that is caused by the fluctuations.

Although the example above is described in detail with the setting ofthe current value in the high resistance state to 0.9 μA and the currentvalue in the low resistance state to 5 μA in addition to the connectionto the load resistor of 5 kΩ, the set current values are not limited tosuch. For example, FIG. 13 shows a case where the current value in thelow resistance state is set to 6.7 μA and the current value in the highresistance state is set to 1.7 μA and FIG. 14 shows a case where thecurrent value in the low resistance state is set to 3.3 μA and thecurrent value in the high resistance state is set to 0.48 μA. Both ofthe cases show that the influence of the fluctuations is probably verylarge when the determination point is set between the current value inthe low resistance state and the current value in the high resistancestate. Thus, setting the determination point to a value closer to thecurrent value in the low resistance state than the median value enablesreduction in the error in reading data that is cased by the influence offluctuations.

Embodiment 3

Next, Embodiment 3 will describe an example of a nonvolatile storagedevice according to the present invention, that is, a 1T1R nonvolatilestorage device.

Structure of Nonvolatile Storage Device

FIG. 15 is a block diagram illustrating an example configuration of anonvolatile storage device 300 according to Embodiment 3 in the presentinvention. As illustrated in FIG. 15, the nonvolatile storage device 300includes a memory cell array 301 including nonvolatile storage elementsR311 to R322, an address buffer 302, a control unit 303, a row decoder304, a word line driver 305, a column decoder 306, and a bit line/plateline driver 307. Furthermore, the bit line/plate line driver 307includes a sensing circuit (sense amplifier) capable of measuring(calculating) a current flowing through a bit line or a plate line, avoltage generated, or a resistance value calculated from the current andthe voltage.

As illustrated in FIG. 15, the memory cell array 301 includes: two wordlines W1 and W2 that extend vertically; two bit lines B1 and B2 thatextend horizontally and intersect with the word lines W1 and W2; twoplate lines P1 and P2 that extend horizontally and correspond to the bitlines B1 and B2, respectively; and four memory cells MC311, MC312,MC321, and MC322 that are arranged in a matrix and correspond tointersections of the word lines W1 and W2 and the bit lines B1 and B2.The memory cells MC311, MC312, MC321, and MC322 include a selectiontransistor T311 and a nonvolatile storage element R311, a selectiontransistor T312 and a nonvolatile storage element R312, a selectiontransistor T321 and a nonvolatile storage element R321, and a selectiontransistor T322 and a nonvolatile storage element R322, respectively.

The number of each of the constituent elements is not limited to theabove. Although the memory cell array 301 includes, for example, thefour memory cells MC311, MC312, MC321, and MC322, it may include five ormore memory cells.

In the example configuration, although the plate lines are arranged inparallel with the bit lines, the plate lines may be arranged in parallelwith the word lines. Furthermore, although the plate lines provide theconnected transistors with a common potential, the plate lines mayinclude a source line selection circuit or a driver having the sameconfiguration as those of the row decoder 304 and the word line driver305, and drive a selected source line and a non-selected source linewith different voltages (including polarities).

Each of the nonvolatile storage elements R311, R312, R321, and R322corresponds to the nonvolatile storage elements 100 and 201 described inEmbodiments 1 and 2. Additionally, in the configuration of the memorycell array 301, the memory cell MC311 (selection transistor T311 andnonvolatile storage element R311) is between the bit line B1 and theplate line P1. In the memory cell MC311, the source of the selectiontransistor T311 is connected in series with the nonvolatile storageelement R311. More specifically, the selection transistor T311 isconnected to the bit line B1 and the nonvolatile storage element R311 inbetween them, and the nonvolatile storage element R311 is connected tothe selection transistor T311 and the plate line P1 in between them.Furthermore, the gate of the selection transistor T311 is connected tothe word line W1.

The connection states of the other three selection transistors T312,T321, and T322 and the three nonvolatile storage elements R312, R321,and R322 that are arranged in series with the selection transistorsT312, T321, and T322, respectively are the same as that of the selectiontransistor T311 and the nonvolatile storage element R311, and thus thedescription is omitted.

With the above configuration, when a predetermined voltage (activationvoltage) is applied to gates of the selection transistors T311, T312,T321, and T322 via the word lines W1 and W2, conduction between a drainand a source of each of the selection transistors T311, T312, T321, andT322 is achieved.

The address buffer 302 receives an address signal ADDRESS from anexternal circuit (not shown), and then, based on the received addresssignal ADDRESS, provides a row address signal ROW to the row decoder 304and a column address signal COLUMN to the column decoder 306. Here, theaddress signal ADDRESS indicates an address of a memory cell selectedfrom among the memory cells MC311, MC312, MC321, and MC322. In addition,the row address signal ROW indicates an address of a row from among theaddresses indicated by the address signals ADDRESS. Similarly, thecolumn address signal COLUMN indicates an address of a column.

The control unit 303 selects one of a write mode, an erase mode, and aread mode, based on a mode selection signal MODE received from theexternal circuit, and performs control corresponding to the selectedmode. According to Embodiment 3, the write mode is a mode in which anonvolatile storage element is set to the low resistance state, theerase mode is a mode in which a nonvolatile storage element is set tothe high resistance state, and the read mode is a mode in which data isread from a nonvolatile storage element (resistance state of thenonvolatile storage element is determined). Here, each voltage isapplied with respect to a potential of a plate line.

Furthermore, in the write mode, the control unit 303 issues a controlsignal CONT instructing to “apply a write voltage” to the bit line/plateline driver 307, in response to input data Din received from theexternal circuit.

Furthermore, in the read mode, the control unit 303 issues, to the bitline/plate line driver 307, a control signal CONT instructing to “applya read voltage”. The control unit 303 further receives, in the readmode, a signal I_(READ) from the bit line/plate line driver 307(detecting), and provides the external circuit with output data Doutindicating a bit value corresponding to the signal I_(READ)(determining). The signal I_(READ) indicates a current value of acurrent that flows through the plate lines P1 and P2 in the read mode.

In the erase mode, the control unit 303 provides a control signal CONTinstructing to “apply an erase voltage”, to the bit line/plate linedriver 307.

The row decoder 304 receives the row address signal ROW provided by theaddress buffer 302, and based on the row address signal ROW, selects oneof the two word lines W1 and W2. Based on the output signal of the rowdecoder 304, the word line driver 305 applies an activation voltage tothe word line selected by the row decoder 304.

The column decoder 306 receives the column address signal COLUMN outputfrom the address buffer 302, and based on the column address signalCOLUMN, selects one of the two bit lines B1 and B2 and also selects oneof the two plate lines P1 and P2 corresponding to the selected bit line.

Upon receipt of the control signal CONT instructing to “apply a writevoltage” from the control unit 303, the bit line/plate line driver 307applies, based on an output signal from the column decoder 306, a writevoltage V_(WRITE) (writing voltage puke) between the bit line and theplate line that are selected by the column decoder 306.

Furthermore, upon receipt of the control signal CONT instructing to“apply a read voltage” from the control unit 303, the bit line/plateline driver 307 similarly applies, based on an output signal from thecolumn decoder 306, a read voltage V_(READ) between the bit line and theplate line that are selected by the column decoder 306. Then, the bitline/plate line driver 307 provides the control unit 303 with the signalI_(READ) indicating the current value of a current that flows throughthe plate line.

Furthermore, upon receipt of the control signal CONT instructing to“apply an erase voltage” from the control unit 303, the bit line/plateline driver 307 applies, based on an output signal from the columndecoder 306, an erase voltage V_(RESET) (erasing voltage pulse) betweenthe bit line and the plate line that are selected by the column decoder306.

Here, the voltage value of the write voltage V_(WRITE) is set, forexample, to −2.4 V with the pulse width of 100 ns. Furthermore, thevoltage value of the read voltage V_(READ) is set, for example, to +0.4V. Furthermore, the voltage value of the erase voltage V_(RESET) is set,for example, to +1.8 V with the pulse width of 100 ns.

Operations of Nonvolatile Storage Device

The following will describe example operations of the nonvolatilestorage device 300 with the configuration, for each of the write mode,the erase mode, and the read mode.

In the following description, when the nonvolatile storage element is inthe low resistance state, the input data Din that the control unit 303receives from the external circuit is represented by “1”. Furthermore,when the nonvolatile storage element is in the high resistance state,the input data Din is represented by “0”.

The address signal ADDRESS is assumed to be a signal indicating anaddress of the memory cell MC311 for the sake of convenience.

Write Mode

The control unit 303 receives the input data Din from the externalcircuit. Here, the control unit 303 issues a control signal CONTinstructing to “apply a write voltage”, to the bit line/plate linedriver 307 when the input data Din indicates “1”. On the other hand, thecontrol unit 303 does not issue the control signal CONT when the inputdata Din indicates “0”.

Upon receipt of the control signal CONT instructing to “apply a writevoltage” from the control unit 303, the bit line/plate line driver 307applies a voltage V_(WRITE) (writing voltage pulse) between the bit lineB1 and the plate line P1 that are selected by the column decoder 306.

Here, the word line driver 305 applies an activation voltage to the wordline W1 selected by the row decoder 304. Thus, conduction between thedrain and the source of the selection transistor T311 is achieved.

As a result, the write voltage V_(WRITE), that is, the writing voltagepulse whose voltage value is −2.4 V with the pulse width of 100 ns isoutput to the plate line with respect to the bit line, and is applied tothe memory cell MC311. Accordingly, a pulse voltage applying unitperforms a write process, which causes the resistance state of thenonvolatile storage element R311 in the memory cell MC311 to change fromthe high resistance state to the low resistance state. On the otherhand, the writing voltage pulse is not applied to the memory cells MC321and MC322 and the activation voltage is not applied to the gate of theselection transistor T312 of the memory cell MC312. Thus, the resistancestates of the nonvolatile storage elements included in the memory cellsMC312, MC321, and MC322 do not change.

As described above, it is possible to change only the nonvolatilestorage element R311 into the low resistance state. Thus, the dataindicating “1” corresponding to the low resistance state is written inthe memory cell MC311.

When the writing into the memory cell MC311 is completed, a new addresssignal ADDRESS is provided to the address buffer 302 and the operationof the nonvolatile storage device 300 in the write mode is repeatedlyperformed on the memory cells other than the memory cell MC311.

Read Mode

The control unit 303 issues, to the bit line/plate line driver 307, acontrol signal CONT instructing to “apply a read voltage”.

Upon receipt of the control signal CONT instructing to “apply a readvoltage” from the control unit 303, the bit line/plate line driver 307applies a read voltage V_(READ) between the bit line B1 and the plateline P1 that are selected by the column decoder 306.

Here, the word line driver 305 applies an activation voltage to the wordline W1 selected by the row decoder 304. Thus, conduction between thedrain and the source of the selection transistor T311 is achieved.

Thus, for example, a measured voltage having a voltage value of +0.4 Vserving as the read voltage V_(READ) is output to the plate line withrespect to the bit line, and is applied to the memory cell MC311.Accordingly, a read current corresponding to the resistance value of thenonvolatile storage element R311 flows from the bit line B1 to the plateline P1 via the nonvolatile storage element R312. The read voltageV_(READ) is a voltage low enough to have no change in the resistancevalue of a variable resistance element of a memory cell with applicationof the voltage to the memory cell.

Since no measured voltage is applied to the memory cells MC321 and MC322and no activation voltage is applied to the gate of the selectiontransistor T312 of the memory cell MC312, the current does not flowthrough the memory cells MC312, MC321, and MC322.

Next, the sense amplifier (not illustrated) connected to the bit linesoutputs, to the control unit 303, the signal I_(READ) indicating acurrent value of the read current that flows through the bit line B1. Inother words, the control unit 303 detects the current that flows througha nonvolatile storage element (detecting).

The control unit 303 determines and outputs, to the outside of thenonvolatile storage device 300, the output data Dout corresponding tothe current value indicated by the signal I_(READ). For example, whenthe current value indicated by the signal I_(READ) is equal to thecurrent value of the current that flows when the nonvolatile storageelement R311 is in the low resistance state, the control unit 303provides the output data Dout that indicates “1”.

Accordingly, the current corresponding to the resistance value of thenonvolatile storage element R311 of the memory cell MC311 flows only tothe memory cell MC311, and then flows from the bit line B1 to the plateline P1. Accordingly, the data indicating “1” is read from the memorycell MC311.

The details of the control procedure in the read mode are shown in (a)of FIG. 16. (a) of FIG. 16 is a flow chart indicating the main procedurefor reading data by the control unit 303. First, the control unit 303detects a current that flows through a nonvolatile storage element byreading the signal (current value) I_(READ) from the bit line/plate linedriver 307, as the detecting (S10). Next, the control unit 303 compares,as the determining, the current value I_(read) detected in the detectingof S10 with the current reference level Iref that is defined by(IRL+IRH)/2<Iref<IRL, where IRL denotes the current that flows throughthe nonvolatile storage element in the low resistance state, and IRH(<IRL) denotes the current that flows through the nonvolatile storageelement in the high resistance state (S11). As a result, it isdetermined that the nonvolatile storage element is in the highresistance state when the current value I_(read) is smaller than thecurrent reference level Iref (S12), and that the nonvolatile storageelement is in the low resistance state when the current value I_(read)is larger than the current reference level Iref (S13). When the currentvalue I_(read) is equal to the current reference level Iref, it may bedetermined that the nonvolatile storage element is in one of the highresistance state and the low resistance state. With the procedure, theresistance state of the nonvolatile storage element can be stablydetermined, based on the current value of a current that flows throughthe nonvolatile storage element.

In the example of (a) of FIG. 16, the control unit 303 determines theresistance state of the nonvolatile storage element, based on thecurrent value I_(read) of the current that flows through the nonvolatilestorage element with application of a fixed voltage. However, when thesignal output from the bit line/plate line driver 307 to the controlunit 303 is the signal indicating the resistance value (resistance valueR_(read)) of the nonvolatile storage element, the resistance state ofthe nonvolatile storage element may be determined based on theresistance value R_(read). (b) of FIG. 16 is a flow chart indicating themain procedure for reading data by the control unit 303, when theresistance state of a nonvolatile storage element is determined based onthe resistance value of the nonvolatile storage element.

First, the control unit 303 detects a resistance value of a nonvolatilestorage element by reading a signal (here, resistance value R_(read))from the bit line/plate line driver 307, as the detecting (S20). Next,the control unit 303 compares, as the determining, the resistance valueR_(read) detected in the detecting of S20 with the resistance referencelevel Rref that is defined by RL<Rref<(RL+RH)/2, where RL denotes theresistance value of the nonvolatile storage element in the lowresistance state, and RH (>RL) denotes the resistance value of thenonvolatile storage element in the high resistance state (S21). As aresult, it is determined that the nonvolatile storage element is in thelow resistance state when the resistance value R_(read) is smaller thanthe resistance reference level Rref (S22), and that the nonvolatilestorage element is in the high resistance state when the resistancevalue R_(read) is larger than the resistance reference level Rref (S23).With the procedure, the resistance state of the nonvolatile storageelement can be stably determined, based on the resistance value of thenonvolatile storage element.

Instead of measuring the resistance value of the nonvolatile storageelement R311 of the memory cell MC311, a voltage in a process where thevoltage pre-charged in the nonvolatile storage element R311 isattenuated with the time constant corresponding to the resistance valueof the nonvolatile storage element R311 may be measured.

When the reading from the memory cell MC311 is completed, a new addresssignal ADDRESS is provided to the address buffer 302 and the operationof the nonvolatile storage device 300 in the read mode is repeatedlyperformed on the memory cells other than the memory cell MC311.

Erase Mode

In the erase mode, the control unit 303 outputs a control signal CONTinstructing to “apply an erase voltage”, to the bit line/plate linedriver 307.

Upon receipt of the control signal CONT instructing to “apply an erasevoltage” from the control unit 303, the bit line/plate line driver 307applies an erase voltage V_(RESET) (erasing voltage pulse) between thebit line B1 and the plate line P1 that are selected by the columndecoder 306.

Here, the word line driver 305 applies an activation voltage to the wordline W1 selected by the row decoder 304. Thus, conduction between thedrain and the source of the selection transistor T311 is achieved.

As a result, the erase voltage V_(RESET), that is, the erasing voltagepulse whose voltage value is +1.8 V with the pulse width of 100 ns isoutput to the plate line with respect to the bit line, and is applied tothe memory cell MC311. Accordingly, a pulse voltage applying unitperforms an erase process, which causes the resistance state of thenonvolatile storage element R311 of the memory cell MC311 to change fromthe low resistance state to the high resistance state. On the otherhand, the erasing voltage pulse is not applied to the memory cells MC321and MC322 and the activation voltage is not applied to the gate of theselection transistor T312 of the memory cell MC312. Thus, the resistancestates of the nonvolatile storage elements included in the memory cellsMC312, MC321, and MC322 do not change.

Although the control unit 303 performs the detecting and the determiningin the read mode according to Embodiment 3, the sense amplifier includedin the bit line/plate line driver 307 or the sense amplifierindependently provided may perform the detecting or both the detectingand the determining instead of the control unit 303. In other words, thecontrol unit 303 and the sense amplifier may appropriately shareprocessing for the detecting and the determining. Here, although theabsolute value of the erase voltage V_(RESET) to be applied to a memorycell is smaller than the absolute value of the write voltage V thetransistor in writing data is in a source-follower connection. Since theON resistance of the transistor in writing the data is higher than thatof the transistor in erasing data, the absolute value of the voltage tobe applied to the variable resistance element in erasing the data islarger than that in the writing.

Embodiment 4

Next, Embodiment 4 will describe another example of a nonvolatilestorage device according to the present invention, that is, across-point nonvolatile storage device. Here, the cross-pointnonvolatile storage device is a storage device in which active layersare provided at points of intersection (three-dimensional cross-points)of word lines and bit lines.

Configuration of Nonvolatile Storage Device

FIG. 17 is a block diagram illustrating an example configuration of anonvolatile storage device 400 according to Embodiment 4 in the presentinvention. As illustrated in FIG. 17, the cross-point nonvolatilestorage device 400 includes a memory cell array 401 includingnonvolatile storage elements R11 to R33, an address buffer 402, acontrol unit 403, a row decoder 404, a word line driver 405, a columndecoder 406, and a bit line driver 407. Furthermore, the bit line driver407 includes a sensing circuit capable of measuring (calculating) acurrent flowing through a bit line, a voltage generated, or a resistancevalue calculated from the current and the voltage.

As illustrated in FIG. 17, the memory cell array 401 includes word linesW1, W2, W3, . . . that extend vertically and are parallel to each other,and bit lines B1, B2, B3, . . . that intersect with the word lines W1,W2, W3, . . . , extend horizontally, and are parallel to each other.Here, the word lines W1, W2, W3, . . . are arranged in a first planeparallel to the main plane of a substrate (not illustrated), and the bitlines B1, B2, B3, . . . are arranged in a second plane positioned aboveor below the first plane and substantially parallel to the first plane.Thus, the word lines W1, W2, W3, . . . three-dimensionally cross the bitlines B1, B2, B3, . . . , and memory cells MC11, MC12, MC13, MC21, MC22,MC23, MC31, MC32, MC33, . . . (hereinafter referred to as “memory cellsMC11, MC12, . . . ”) are provided for the correspondingthree-dimensional cross-points.

Each of the memory cells MC11, MC12, . . . includes a corresponding oneof the nonvolatile storage elements R11, R12, R13, R21, R22, R23, R31,R32, R33, . . . that are connected in series, and a corresponding one ofcurrent steering elements D11, D12, D13, D21, D22, D23, D31, D32, andD33, . . . each including, for example, a bidirectional diode. Thenonvolatile storage elements R11, R12, R13, R21, R22, R23, R31, R32,R33, . . . are connected to the bit lines B1, B2, B3, . . . , and thecurrent steering elements D11, D12, D13, D21, D22, D23, D31, D32, andD33, . . . are connected to the nonvolatile storage elements and theword lines W1, W2, W3, . . . . The nonvolatile storage elements 100 and201 according to Embodiments 1 and 2 may be used as the nonvolatilestorage elements R11, R12, R13, R21, R22, R23, R31, R32, R33, . . . .Furthermore, a metal insulator metal (MIM) diode, a metal semiconductormetal (MSM) diode, a varistor, or others may be used as the currentsteering elements D11, D12, D13, D21, D22, D23, D31, D32, and D33, . . ..

The address buffer 402 receives an address signal ADDRESS from anexternal circuit (not illustrated), and then, based on the receivedaddress signal ADDRESS, provides a row address signal ROW to the rowdecoder 404 and a column address signal COLUMN to the column decoder406. Here, the address signal ADDRESS indicates an address of a memorycell to be selected from among the memory cells MC12, MC21, . . . . Inaddition, the row address signal ROW indicates an address of a row fromamong the addresses indicated by the address signals ADDRESS. Similarly,the column address signal COLUMN indicates an address of a column.

Here, each voltage is applied with respect to the bit line.

The control unit 403 selects one of a write mode, an erase mode, and aread mode, based on a mode selection signal MODE received from theexternal circuit, and performs control corresponding to the selectedmode.

Furthermore, in the write mode and the erase mode, the control unit 403applies a write voltage pulse and an erasing voltage pulse,respectively, to the word line driver 405, according to input data Dinreceived from the external circuit.

Furthermore, in the read mode, the control unit 403 applies a readvoltage to the word line driver 405. The control unit 403 furtherreceives, in the read mode, a signal I_(READ) from the bit line driver407 (detecting), and provides the external circuit with output data Doutindicating a bit value corresponding to the signal I_(READ)(determining). The signal i_(READ) indicates a current value of acurrent that flows through the word lines W1, W2, W3, . . . in the readmode.

The row decoder 404 receives the row address signal ROW output from theaddress buffer 402, and based on the row address signal ROW, selects oneof the word lines W1, W2, W3, . . . . Based on the output signalreceived by the row decoder 404, the word line driver 405 applies anactivation voltage to the word line selected by the row decoder 404.

The column decoder 406 receives the column address signal COLUMN outputfrom the address buffer 402, and selects one of the bit lines B1, B2,B3, . . . , based on the column address signal COLUMN.

The bit line driver 407 connects the bit line selected by the columndecoder 406 to ground, based on the output signal received by the columndecoder 406.

Although Embodiment 4 describes a cross-point nonvolatile storage devicehaving a single layer, a cross-point nonvolatile storage device havingmultiple layers may be employed by stacking memory cell arrays.

In addition, the nonvolatile storage elements and the current steeringelements may exchange the positional relationship. More specifically,the bit lines and the word lines may be connected to the nonvolatilestorage elements and the current steering elements, respectively.

In addition, one or both of the bit lines and the word lines may alsoserve as electrodes in the nonvolatile storage elements.

Operations of Nonvolatile Storage Device

The following will describe an example operation of the nonvolatilestorage device 400 with the configuration, for each of the write mode,the erase mode, and the read mode. Since known methods can be used forselecting a bit line and a word line and for applying a voltage pulse,the detail description is omitted.

In the following example, writing and reading are performed on thememory cell MC22. Since the ON resistance of a current steering element(diode) included in a memory cell is higher than that of a transistor,the voltage to be applied to the memory cell in each of the write mode,the erase mode, and the read mode is higher than that of a memory cellincluding a transistor.

Write Mode

When data indicating “1” is written (stored) into the memory cell MC22,the bit line driver 407 connects the bit line B2 to ground, and the wordline driver 405 electrically connects the word line W2 to the controlunit 403. Then, the control unit 403 applies a write voltage pulse tothe word line W2. The voltage value of the writing voltage pulse is setto −4.0 V with the pulse width of 100 ns, for example. The writingvoltage pulse is a voltage capable of turning ON the current steeringelement, and is an application voltage serving as a write voltage to setthe variable resistance element in the low resistance state.

With the operations, a pulse voltage applying unit performs a writeprocess for applying the writing voltage pulse to the nonvolatilestorage element R22 of the memory cell MC22, which causes thenonvolatile storage element R22 in the memory cell MC22 to change to thelow resistance state corresponding to “1”.

Erase Mode

In writing (erasing) data indicating “0” to the memory cell MC22, thebit line driver 407 connects the bit line B2 to ground, and the wordline driver 405 electrically connects the word line W2 to the controlunit 403. Then, the control unit 403 applies the erasing voltage pulseto the word line W2. Here, for example, the voltage value of the erasingvoltage pulse is set to +5.0 V with the pulse width of 100 ns. The erasevoltage turns ON the current steering element, and is a voltage havingan absolute value larger than that of the write voltage to set thevariable resistance element to the high resistance state.

With the operations, a pulse voltage applying unit performs an eraseprocess for applying the erasing voltage pulse to the nonvolatilestorage element R22 of the memory cell MC22, which causes thenonvolatile storage element R22 to change to the high resistance statecorresponding to “0”.

Read Mode

In writing data written to the memory cell MC22, the bit line driver 407connects the bit line B2 to ground, and the word line driver 405electrically connects the word line W2 to the control unit 403. Then,the control unit 403 applies a read voltage to the word line W2. Here,for example, the voltage value of the read voltage is set to +2.8 V. Theread voltage turns ON the current steering element, and is a readvoltage to cause no resistance change to the variable resistanceelement.

With application of the read voltage to the memory cell MC22, thecurrent having the current value corresponding to the resistance valueof the nonvolatile storage element R22 of the memory cell MC22 flowsbetween the bit line B2 and the word line W2. The control unit 403detects a current value of the current (detecting), and detects theresistance state of memory cell MC22, based on the current value and theread voltage (determining).

When the nonvolatile storage element R22 of the memory cell MC22 is inthe low resistance state, it is determined that the data written to thememory cell MC22 is “1”. Furthermore, when the nonvolatile storageelement R22 is in the high resistance state, it is determined that thedata written to the memory cell MC22 is “0”.

The details of the control procedure in the read mode are shown in (a)and (b) of FIG. 16. The nonvolatile storage element according toEmbodiment 4 differs from the 1T1R nonvolatile storage element accordingto Embodiment 3 only in that the word line driver 405 transmits a signalindicating one of (i) the current value I_(READ) of a current that flowsto the nonvolatile storage element and (ii) the resistance valueR_(READ) of the nonvolatile storage element. The other processing(detecting, determining, etc.) of the control unit 403 in the read modeis the same as that of the control unit 303 according to Embodiment 3.

Although a bit line is grounded and a predetermined voltage pulse isapplied to a word line in the description above, voltage pulsesdifferent from each other may be applied to the bit line and the wordline so that the potential difference becomes a predetermined voltage.

Although the control unit 403 performs the detecting and the determiningin the read mode according to Embodiment 4, the sense amplifier includedin the word line driver 405 or a sense amplifier independently providedmay perform the detecting or both the detecting and the determininginstead of the control unit 403, In other words, the control unit 403and the sense amplifier may appropriately share processing for thedetecting and the determining.

Since the method for reading data and the nonvolatile storage deviceaccording to the present invention are described based on Embodiments 1to 4, the present invention is not limited to these Embodiments. Variousmodifications which can be conceived by those skilled in the art withoutmaterially departing from the novel teachings and advantages of thepresent invention are intended to be included within the scope of thepresent invention.

Although Embodiments 1 to 4 describe determining a resistance state of anonvolatile storage element, based on a current value of a current thatflows through the nonvolatile storage element with application of afixed voltage or based on a resistance value of the nonvolatile storageelement, the present invention is not limited to the determinationmethod based on such information.

For example, the resistance state of the nonvolatile storage element maybe determined based on a voltage (that is, voltage drop) in thenonvolatile storage element with application of a fixed current, orbased on the time constant defined by the nonvolatile storage elementand a capacitor of a fixed capacitance (or a counter value indicating atime corresponding to the time constant). In any of the methods, theadvantages of the present invention can be produced by setting thereference level to be referenced in determining a resistance state ofthe nonvolatile storage element to a value closer to a physical value ofthe nonvolatile storage element in the low resistance state than amedian value between the physical value of the nonvolatile storageelement in the low resistance state and a physical value of thenonvolatile storage element in the high resistance state.

INDUSTRIAL APPLICABILITY

The present invention is applicable to nonvolatile storage elements usedin, particularly, various electronic devices, such as digital homeappliances, memory cards, mobile phones, and personal computers, as amethod for reading data from a variable resistance nonvolatile storageelement whose resistance value changes according to an electric signalto be applied, and as the nonvolatile storage device.

REFERENCE SIGNS LIST

100, 201, R11 to R33, R311 to R322 Variable resistance nonvolatilestorage element

101 Substrate

102 Oxide layer

103 Lower electrode

104 First transition metal oxide layer (first oxygen-deficient tantalumoxide layer, first oxygen-deficient hafnium oxide layer, firstoxygen-deficient zirconium oxide layer, etc.)

105 Second transition metal oxide layer (second tantalum oxide layer,second hafnium oxide layer, second zirconium oxide layer, titanium oxidelayer, etc.)

106 Variable resistance layer

107 Upper electrode

202 Load resistor

203 Terminal

204 Terminal

300, 400 Nonvolatile storage device

301, 401 Memory cell array

302, 402 Address buffer

303, 403 Control unit

304, 404 Row decoder

305, 405 Word line driver

306, 406 Column decoder

307 Bit line/plate line driver

407 Bit line driver

MC11 to MC33, MC311 to MC322 Memory cell

T311 to T322 Selection transistor

D11 to D33 Current steering element

1-22. (canceled)
 23. A method for reading data from a variableresistance nonvolatile storage element (i) including a first electrode,a second electrode, and a variable resistance layer disposed between andin contact with the first electrode and the second electrode and (ii)having characteristics in which a resistance state between the firstelectrode and the second electrode with application of a voltage havinga first polarity between the first electrode and the second electrodebecomes a first resistance state RL, and in which the resistance statebetween the first electrode and the second electrode with application ofa voltage having a second polarity different from the first polaritybetween the first electrode and the second electrode becomes a secondresistance state RH, the second resistance state RH >the firstresistance state RL, the nonvolatile storage element being an elementhaving fluctuations that are characteristics in which a resistance valueof the nonvolatile storage element in the second resistance state RHrandomly changes with passage of time, the method comprising: detectinga current that flows through the nonvolatile storage element withapplication of a fixed voltage; and determining that (i) the nonvolatilestorage element is in a high resistance state when the current detectedin the detecting is smaller than a current reference level Iref, and(ii) the nonvolatile storage element is in a low resistance state whenthe current detected in the detecting is larger than the reference levelIref, the current reference level Iref being defined by(IRL+IRH)/2<Iref<IRL, where IRL denotes a current that flows through thenonvolatile storage element in the first resistance state RL withapplication of the fixed voltage, IRH denotes a current that flowsthrough the nonvolatile storage element in the second resistance stateRH, and IRH<IRL.
 24. The method according to claim 23, wherein in thedetermining, a current value larger than an average value of thefluctuations by at least 4σ is determined as the current reference levelIref satisfying (IRL+IRH)/2<Iref<IRL, where σ denotes a standarddeviation in the fluctuations of the current value IRH of thenonvolatile storage element in the second resistance state RH.
 25. Amethod for reading data from a variable resistance nonvolatile storageelement (i) including a first electrode, a second electrode, and avariable resistance layer disposed between and in contact with the firstelectrode and the second electrode and (ii) having characteristics inwhich a resistance state between the first electrode and the secondelectrode with application of a voltage having a first polarity betweenthe first electrode and the second electrode becomes a first resistancestate RL, and in which the resistance state between the first electrodeand the second electrode with application of a voltage having a secondpolarity different from the first polarity between the first electrodeand the second electrode becomes a second resistance state RH, thesecond resistance state RH>the first resistance state RL, thenonvolatile storage element being an element having fluctuations thatare characteristics in which a resistance value of the nonvolatilestorage element in the second resistance state RH randomly changes withpassage of time, the method comprising: detecting a resistance value ofthe nonvolatile storage element; and determining that (i) thenonvolatile storage element is in a low resistance state when theresistance value detected in the detecting is smaller than a resistancereference level Rref, and (ii) the nonvolatile storage element is in ahigh resistance state when the resistance value detected in thedetecting is larger than the resistance reference level Rref, theresistance reference level Rref being defined by RL<Rref<(RL+RH)/2. 26.The method according to claim 25, wherein in the determining, aresistance value smaller than an average value of the fluctuations by atleast 4σ is determined as the resistance reference level Rref satisfyingRL<Rref<(RL+RH)/2, where σ denotes a standard deviation in thefluctuations of the resistance value of the nonvolatile storage elementin the second resistance state RH.
 27. The method according to claim 23,wherein the variable resistance layer has a stacked structure including(i) a first transition metal oxide comprising a first transition metaland (ii) a second transition metal oxide comprising a second transitionmetal, the first transition metal oxide being higher in oxygendeficiency than the second transition metal oxide.
 28. The methodaccording to claim 27, wherein the second transition metal oxide islarger in resistance value than the first transition metal oxide. 29.The method according to claim 27, wherein the first transition metaloxide is identical to the second transition metal oxide.
 30. The methodaccording to claim 29, wherein the first transition metal oxide and thesecond transition metal oxide comprise tantalum.
 31. The methodaccording to claim 27, wherein the first transition metal oxide isdifferent from the second transition metal oxide, and the secondtransition metal oxide is lower in standard electrode potential than thefirst transition metal oxide.
 32. A nonvolatile storage device,comprising: a variable resistance nonvolatile storage element; and acontrol unit configured to read data from the nonvolatile storageelement, wherein the nonvolatile storage element (i) includes a firstelectrode, a second electrode, and a variable resistance layer disposedbetween and in contact with the first electrode and the second electrodeand (ii) has characteristics in which a resistance state between thefirst electrode and the second electrode with application of a voltagehaving a first polarity between the first electrode and the secondelectrode becomes a first resistance state RL, and in which theresistance state between the first electrode and the second electrodewith application of a voltage having a second polarity different fromthe first polarity between the first electrode and the second electrodebecomes a second resistance state RH, the second resistance state RH>thefirst resistance state RL, the nonvolatile storage element being anelement having fluctuations that are characteristics in which aresistance value of the nonvolatile storage element in the secondresistance state RH randomly changes with passage of time, and thecontrol unit is configured to: detect a current that flows through thenonvolatile storage element with application of a fixed voltage; anddetermine that (i) the nonvolatile storage element is in a highresistance state when the detected current is smaller than a currentreference level Iref, and (ii) the nonvolatile storage element is in alow resistance state when the detected current is larger than thereference level Iref, the current reference level Iref being defined by(IRL+IRH)/2<Iref<IRL, where IRL denotes a current that flows through thenonvolatile storage element in the first resistance state RL withapplication of the fixed voltage, IRH denotes a current that flowsthrough the nonvolatile storage element in the second resistance stateRH, and IRH<IRL.
 33. The nonvolatile storage device according to claim32, wherein the control unit is configured to determine a current valuelarger than an average value of the fluctuations by at least 4σ as thecurrent reference level Iref satisfying (IRL+IRH)/2<Iref<IRL, where σdenotes a standard deviation in the fluctuations of the current valueIRH of the nonvolatile storage element in the second resistance stateRH.
 34. A nonvolatile storage device, comprising: a variable resistancenonvolatile storage element; and a control unit configured to read datafrom the nonvolatile storage element, wherein the nonvolatile storageelement (i) includes a first electrode, a second electrode, and avariable resistance layer disposed between and in contact with the firstelectrode and the second electrode and (ii) has characteristics in whicha resistance state between the first electrode and the second electrodewith application of a voltage having a first polarity between the firstelectrode and the second electrode becomes a first resistance state RL,and in which the resistance state between the first electrode and thesecond electrode with application of a voltage having a second polaritydifferent from the first polarity between the first electrode and thesecond electrode becomes a second resistance state RH, the secondresistance state RH>the first resistance state RL, the nonvolatilestorage element being an element having fluctuations that arecharacteristics in which a resistance value of the nonvolatile storageelement in the second resistance state RH randomly changes with passageof time, and the control unit is configured to: detect a resistancevalue of the nonvolatile storage element; and determine that (i) thenonvolatile storage element is in a low resistance state when thedetected resistance value is smaller than a resistance reference levelRref, and (ii) the nonvolatile storage element is in a high resistancestate when the detected resistance value is larger than the resistancereference level Rref, the resistance reference level Rref being definedby RL<Rref<(RL+RH)/2.
 35. The nonvolatile storage device according toclaim 34, wherein the control unit is configured to determine aresistance value smaller than an average value of the fluctuations by atleast 4σ as the resistance reference level Rref satisfyingRL<Rref<(RL+RH)/2, where σ denotes a standard deviation in thefluctuations of the resistance value of the nonvolatile storage elementin the second resistance state RH.
 36. The nonvolatile storage deviceaccording to claim 32, wherein the variable resistance layer has astacked structure including (i) a first transition metal oxidecomprising a first transition metal and (ii) a second transition metaloxide comprising a second transition metal, the first transition metaloxide being higher in oxygen deficiency than the second transition metaloxide.
 37. The nonvolatile storage device according to claim 36, whereinthe second transition metal oxide is larger in resistance value than thefirst transition metal oxide.
 38. The nonvolatile storage deviceaccording to claim 36, wherein the first transition metal oxide isidentical to the second transition metal oxide.
 39. The nonvolatilestorage device according to claim 38, wherein the first transition metaloxide and the second transition metal oxide comprise tantalum.
 40. Thenonvolatile storage device according to claim 36, wherein the firsttransition metal oxide is different from the second transition metaloxide, and the second transition metal oxide is lower in standardelectrode potential than the first transition metal oxide.
 41. Themethod according to claim 23, wherein the nonvolatile storage elementhas fluctuations that are characteristics in which a resistance value ofthe nonvolatile storage element in the first resistance state RLrandomly changes with passage of time; and the nonvolatile storageelement in the second resistance state RH has the fluctuations inresistance value larger than the fluctuations of the nonvolatile storageelement in the first resistance state RL.
 42. The nonvolatile storagedevice according to claim 32, wherein the nonvolatile storage elementhas fluctuations that are characteristics in which a resistance value ofthe nonvolatile storage element in the first resistance state RLrandomly changes with passage of time; and the nonvolatile storageelement in the second resistance state RH has the fluctuations inresistance value larger than the fluctuations of the nonvolatile storageelement in the first resistance state RL.